COMPAT: Break in openpyxl 
- [ ] 3.4 build is now using current version of `openpyxl` (2.0.2), fix for this, see here: https://travis-ci.org/pydata/pandas/jobs/25510797
- [ ] change ci/requirements-2.7.txt to use last version (1.8.6); will need same import fixes

When I try to use     `DataFrame` `to_excel`, I was given the following error. I am using `Pandas 0.13.1`, `openpyxl 2.0.2`

```
from openpyxl.style import Style
ImportError: No module named style
```
