WARN: matplotlib warnings
anyone know the reason for these?  https://travis-ci.org/pydata/pandas/jobs/111469225
only seem to be on py3.

```
/home/travis/miniconda/envs/pandas/lib/python3.5/site-packages/matplotlib/font_manager.py:273: UserWarning: Matplotlib is building the font cache using fc-list. This may take a moment.
  warnings.warn('Matplotlib is building the font cache using fc-list. This may take a moment.')
/home/travis/miniconda/envs/pandas/lib/python3.5/site-packages/matplotlib/font_manager.py:273: UserWarning: Matplotlib is building the font cache using fc-list. This may take a moment.
  warnings.warn('Matplotlib is building the font cache using fc-list. This may take a moment.')
```
