TST: catch some more warnings
[for example on the 3.5 build](https://travis-ci.org/pydata/pandas/jobs/105685173)

like to `assert_produces_warning` on these to remove from the test output; these are legit.

```
./home/travis/build/pydata/pandas/pandas/indexes/base.py:2385: RuntimeWarning: unorderable types: str() < int(), sort order is undefined for incomparable objects
  return this.join(other, how=how, return_indexers=return_indexers)
```
