Add the pip download cache to the Travis CI cache
Add the pip download cache to the Travis CI cache

This at least lets us cache the downloads and should
speed up the build some n amount.

(sapling split of 49739419ffbc6f67843d405a42a8ec18722cad9d)
foursquare/fsqio,foursquare/fsqio,foursquare/fsqio,foursquare/fsqio,foursquare/fsqio