Faster builds on Travis

[dideler]

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

Just have to add sudo: false to .travis.yml. Unfortunately we can't do that now because we have some (apt) commands that require sudo.

A workaround it suggests is to use precompiled binaries, uploading them to S3 and downloading them as part of your build, installing them into a non-root directory. I think that's too much effort for now, but opening this issue so we're aware of the possibility.