TST: try using the new TravisCI container-based infrastructure.

Using that infrastructure brings more cores and RAM, should be faster.
Requires not using sudo, as documented at http://docs.travis-ci.com/user/workers/container-based-infrastructure/

Let's see if this works and is really faster...

Docs don't describe at all how to cache ``pip`` installed packages or how to not execute part of script when the result of that script is cached in arbitrary dir.