CI: invalidate cache when .pxi.in only changes
In the following, *only* the ``.pxi.in`` were changed and ``setup.py`` correctly has dependencies on these, so local building is fine. However, on travis, the cython cache does not seem to be invalidation (and I had to resort to manually clearing it).

similar to my comment: https://github.com/pandas-dev/pandas/pull/14937#issuecomment-273270706

happened again on merging: #15139, see: https://travis-ci.org/pandas-dev/pandas/jobs/193092807

