travis: request containers

with sudo: false (we don't use sudo) we can use faster builds.

see: http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/
travis: re-enable caching

Available for open source repos now.

See: http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/
Merge pull request #673 from uwescience/travis-docker

travis: upgrade infrastructure