DOC: Handle exceptions when computing contributors.
We're failing on master right now https://travis-ci.org/pandas-dev/pandas/jobs/455255389#L4369

```
Exception occurred:
  File "/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/git/cmd.py", line 825, in execute
    raise GitCommandError(command, status, stderr_value, stdout_value)
git.exc.GitCommandError: Cmd('git') failed due to: exit code(128)
  cmdline: git shortlog -s v0.9.0..v0.10.0
  stderr: 'fatal: ambiguous argument 'v0.9.0..v0.10.0': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]''
The full traceback has been saved in /tmp/sphinx-err-hzfqasu4.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
```

I'm not sure why that is. Perhaps something with how the repo is cloned.

Anyway, this will work around the error.

```
/Users/taugspurger/sandbox/pandas/doc/source/whatsnew/v0.10.0.rst:450: WARNING: Cannot find contributors for range 'v0.9.0..v0.1.0'
looking for now-outdated files... none found
```