Wrong Gym version in Travis cache

[jvmncs]

The current version of Gym in our Travis cache is 0.11.0, however we're using 0.10.9. This causes the build to break as in #23.

We should either modify the code to work with 0.11.0, or delete the current cache. We should also have it check for changes to the setup.py and overwriting the previous cache before restoring pip caches in the future.