Speed up Travis builds

[sylvinus]

Travis builds are timeouting :-/

There are a few things to investigate:

• [x] Cache Docker builds. See https://github.com/docker/docker/issues/20316 https://docs.travis-ci.com/user/caching/ https://developer.zendesk.com/blog/big-and-fast-tests-taking-our-travis-build-from-4-hours-to-13-minutes http://michael.stapelberg.de/Artikel/docker-on-travis-for-new-tools-and-fast-runs for ideas.
• [x] If we add a versioning system to invalidate the Docker cache, it would be great to use it to warn developers that their local images are out of date too.
• [ ] Parallelize tests
• [ ] Optimize longer tests, particularly the new PageRank ones.

[sylvinus]

Last Travis build was 7 minutes :)