Issue title: Simplecov appears to be "forgetting" an earlier named suite
 Issue body: We have a project that uses private travis-ci for our CI builds.  We use simplecov to measure our code coverage and maintain the coverage at 100% (sans some files we choose to filter out when we determine it's not worth the effort to cover them).  In the last few weeks we've had about a dozen travis builds fail with 82.96% coverage rather than the expected 100%.  In each and every case, the build has succeeded with 100% coverage when we retry it.  We've been unable to repro this locally, so we're a bit stumped.

Digging into the travis logs some more, it appears to be a problem with merging the results, or how the sub-suites are named or something.

Here's a truncated form of the output we see when things work (and get 100% coverage):

```
+ export SIMPLECOV_SUITE=unit_and_integration
+ SIMPLECOV_SUITE=unit_and_integration
+ ./bin/rspec spec/unit spec/integration -Ispec -rsimplecov_setup --format progress -rsupport/verify_rspec_fire_constants -b
Run options: include {:f=>true}
All examples were filtered out; ignoring {:f=>true}
..........................................................................................................................................................................................................................................................................................................**........**.............**..**....**............................................................................................................................................................................................................................................................................................................................................................................................................*.*.............................................................................................................................................................................................................................................................................................................................................**.............................*................*..............***....................................................................................................................................................................................................................................***............*...............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................*............................................................................................................................................................................................................................................................................................................................................................
Pending:
# (truncated)
Finished in 1 minute 53.88 seconds
2365 examples, 0 failures, 24 pending
Randomized with seed 64543
Coverage report generated for unit_and_integration to /home/travis/build/seomoz/vanguard/coverage. 9566 / 9897 LOC (96.66%) covered.
+ export SIMPLECOV_SUITE=acceptance_dump_db
+ SIMPLECOV_SUITE=acceptance_dump_db
+ ./bin/rspec spec/acceptance -Ispec -rsimplecov_setup --format progress --tag dump_db -b
Run options: include {:f=>true, :dump_db=>true}
..........
Finished in 5 minutes 8 seconds
10 examples, 0 failures
Randomized with seed 36603
Coverage report generated for acceptance_dump_db, unit_and_integration to /home/travis/build/seomoz/vanguard/coverage. 9580 / 9897 LOC (96.8%) covered.
+ export SIMPLECOV_SUITE=acceptance
+ SIMPLECOV_SUITE=acceptance
+ ./bin/rspec spec/acceptance -Ispec -rsimplecov_setup --format progress --profile --tag '~dump_db' -b
/home/travis/.rvm/rubies/ruby-1.9.3-Run options:
  include {:f=>true}
  exclude {:dump_db=>true}
All examples were filtered out; ignoring {:f=>true}
........................................................*.................*............*..*.....***.****.*.*.
Pending:
# (truncated)
Finished in 3 minutes 15.4 seconds
109 examples, 0 failures, 13 pending
Randomized with seed 63778
Coverage report generated for acceptance, acceptance_dump_db, unit_and_integration to /home/travis/build/seomoz/vanguard/coverage. 9897 / 9897 LOC (100.0%) covered.
```

Compare that to the output we get when the build fails with 82.96% coverage:

```
+ export SIMPLECOV_SUITE=unit_and_integration
+ SIMPLECOV_SUITE=unit_and_integration
+ ./bin/rspec spec/unit spec/integration -Ispec -rsimplecov_setup --format progress --profile -rsupport/verify_rspec_fire_constants -b
Run options: include {:f=>true}
All examples were filtered out; ignoring {:f=>true}
............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................*.......................................................................................................................***............................................................................................................................................................................................................................................................**..................................................................................................*......................................................................................................................*...........*............**..........*.............................................................................................................................................................................................................................................*...................................*....................................................................................................................................................................................................................................................................................................................................................................................................................................................**.....**.....**......**......**...................................................................................................................................................................................................................................................................................................................................................................
Pending:
# (truncated)
Finished in 2 minutes 29.4 seconds
2365 examples, 0 failures, 24 pending
Randomized with seed 31504
Coverage report generated for unit_and_integration to /home/travis/build/seomoz/vanguard/coverage. 9565 / 9897 LOC (96.65%) covered.
+ export SIMPLECOV_SUITE=acceptance_dump_db
+ SIMPLECOV_SUITE=acceptance_dump_db
+ ./bin/rspec spec/acceptance -Ispec -rsimplecov_setup --format progress --profile --tag dump_db -b
Run options: include {:f=>true, :dump_db=>true}
..........
Finished in 7 minutes 39 seconds
10 examples, 0 failures
Randomized with seed 31063
Coverage report generated for acceptance_dump_db, unit_and_integration to /home/travis/build/seomoz/vanguard/coverage. 9580 / 9897 LOC (96.8%) covered.
+ export SIMPLECOV_SUITE=acceptance
+ SIMPLECOV_SUITE=acceptance
+ ./bin/rspec spec/acceptance -Ispec -rsimplecov_setup --format progress --profile --tag '~dump_db' -b
Run options:
  include {:f=>true}
  exclude {:dump_db=>true}
All examples were filtered out; ignoring {:f=>true}
.......*.................................................................*......*..*.*******..*.......*......
Pending:
# (truncated)
Finished in 3 minutes 47.3 seconds
109 examples, 0 failures, 13 pending
Randomized with seed 45273
Coverage report generated for acceptance, acceptance_dump_db to /home/travis/build/seomoz/vanguard/coverage. 7936 / 9566 LOC (82.96%) covered.
```

That's a lot to take in, but I wanted to include it for completeness.  Here's the pertinent bits from when the build succeeds:

```
Coverage report generated for unit_and_integration to /home/travis/build/seomoz/vanguard/coverage. 9566 / 9897 LOC (96.66%) covered.
Coverage report generated for acceptance_dump_db, unit_and_integration to /home/travis/build/seomoz/vanguard/coverage. 9580 / 9897 LOC (96.8%) covered.
Coverage report generated for acceptance, acceptance_dump_db, unit_and_integration to /home/travis/build/seomoz/vanguard/coverage. 9897 / 9897 LOC (100.0%) covered.
```

...and the pertinent bits from when it fails:

```
Coverage report generated for unit_and_integration to /home/travis/build/seomoz/vanguard/coverage. 9565 / 9897 LOC (96.65%) covered.
Coverage report generated for acceptance_dump_db, unit_and_integration to /home/travis/build/seomoz/vanguard/coverage. 9580 / 9897 LOC (96.8%) covered.
Coverage report generated for acceptance, acceptance_dump_db to /home/travis/build/seomoz/vanguard/coverage. 7936 / 9566 LOC (82.96%) covered.
```

Notice that in this case, it has "forgotten" the `unit_and_integration` coverage results.

Here's our build script:

```
#!/bin/bash

set -e -x

rspec_command="bin/rspec"
simplecov_setup="-Ispec -rsimplecov_setup"

export SIMPLECOV_SUITE=unit_and_integration
time $rspec_command spec/unit spec/integration $simplecov_setup --format progress --profile -rsupport/verify_rspec_fire_constants -b
export SIMPLECOV_SUITE=acceptance_dump_db
time $rspec_command spec/acceptance $simplecov_setup --format progress --profile --tag dump_db -b
export SIMPLECOV_SUITE=acceptance
time $rspec_command spec/acceptance $simplecov_setup --format progress --profile --tag ~dump_db -b
```

It loads simplecov via a `spec/simplecov_setup` file, which among a bunch of filters and the `at_exit` configuration has the following:

```
SimpleCov.start do
  command_name ENV['SIMPLECOV_SUITE'] if ENV.has_key?('SIMPLECOV_SUITE')
end
```

Any idea why the `unit_and_integration` suite is being forgotten when it comes time to merge the results?

 Comments: 
 Comment 0: We figured this out -- we need to set the `merge_timeout` to a longer value.  It's been documented well.  I should have RTFM first...

 Comment 1: Haha, nice one :grin: Still thanks for providing a great and detailed bug report :+1: 

{'source': 'GithubIssues', 'repo_name_with_owner': 'simplecov-ruby/simplecov', 'issue_id': 16014048, 'repo_stars': 4518.0}