Switch to container-based infrastructure on Travis CI.

The container-based infrastructure has more memory and more cores. It also is designed to startup faster. We give up the ability to use sudo, but we aren't using that right now so we should be good.
Props tw2113, netweb

Fixes #2736

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5584 9866e705-20ec-0310-96e7-cbb4277adcfb