DOC: styler warnings in doc-build
https://travis-ci.org/pandas-dev/pandas/jobs/222779268

```
/tmp/doc/source/generated/pandas.io.formats.style.Styler.rst:74: WARNING: failed to import template:
/tmp/doc/source/generated/pandas.io.formats.style.Styler.rst:74: WARNING: toctree references unknown document 'generated/template:'
```

cc @TomAugspurger @jorisvandenbossche 

I just pushed a change to fix the path of the imports (after ``pandas.formats`` change), but I think it still needs something.