Use Travis CIs new container based builds.

The announcement is here:
  http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure
The specs here:
  http://docs.travis-ci.com/user/workers/container-based-infrastructure
  http://docs.travis-ci.com/user/caching/#Fetching-and-storing-caches

This also turns on caching for ~/.pants.d, ~/.ivy2
and the current pants version bootstraped in
build-support.  To make virtualenv caching work, updates
the virtualenv bootstrap script to use a stable directory name
across virtualenv versions.