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.

https://rbcommons.com/s/twitter/r/1537/