Enable caching in the Travis CI build.

The specs here:
  http://docs.travis-ci.com/user/caching

This turns on caching for ~/.pants.d, ~/.ivy2
and the current pants requirements bootstraped in
build-support.  To make virtualenv caching work, updates
the virtualenv bootstrap script to use a stable directory name
across virtualenv versions.

Testing Done:
CI went green here: https://travis-ci.org/pantsbuild/pants/builds/44760074

Bugs closed: 887

Reviewed at https://rbcommons.com/s/twitter/r/1537/
Sync of https://github.com/pantsbuild/pants/commit/44043be01259257ebf18a09f50e5d825e4db83e9