[WIP] Improve TravisCI builds

I've talked about this before: [here](https://github.com/rails/rails/issues/17358) and [here](https://groups.google.com/forum/#!topic/rubyonrails-core/reXLhg_Jo5k). Just now in the weekend I can get around to having some tries with the idea :).

I'm submitting this PR as `WIP` because I'm still working on it. These commits need to be run on TravisCI in order to see what differences it makes so I'll remove `WIP` once ready.

This might be a trivial change, yet faster builds = happier contributors, right :laughing:?