Use container-based environment to speed up Travis builds

[lafrech]

I think we could make the Travis CI tests run faster using container-based environment.

Docs: https://docs.travis-ci.com/user/ci-environment/
Blog article: https://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/

Here's umongo's .travis.yml, as an example: https://github.com/Scille/umongo/blob/master/.travis.yml. It shows how to intall MongoDB using addons rather than using sudo apt-get.

[wojcikstefan]

Good call, thanks @lafrech !