.travis.yml: enable the pip cache
.travis.yml: enable the pip cache

This caches `$HOME/.cache/pip` in Travis builds.

Our Travis build time is often dominated by dependency installation,
so caching pip can speed up builds significantly.

The cache can be managed at:
https://travis-ci.org/MechanicalSoup/MechanicalSoup/caches

MechanicalSoup/MechanicalSoup,hickford/MechanicalSoup,hemberger/MechanicalSoup