faster travis builds using containers

[piyush-kurur]

Travis build are faster if one uses the container infrastructure. The
main limitation is that one is not allowed to use sudo. However, hvr's
ppa is available for the container infrastructure See:

• https://docs.travis-ci.com/user/workers/container-based-infrastructure/
• https://github.com/hvr/multi-ghc-travis#travisyml-for-container-based-infrastructure

It might be a good idea to move to this style.

[bergey]

Yes, we should move monoid-extras and all the Diagrams packages to containers. If you have time to make a PR, that would be great.

My last attempt is at https://github.com/diagrams/diagrams-core/compare/travis. I ran out of time to figure out why the build was failing. monoid-extras should be easier because it has no dependencies that need to track git HEAD.