Travis: use Travis-CI's container-based infrastructure.

This may help to improve performance (how quickly it takes for the entire suite of tests to complete).
See http://docs.travis-ci.com/user/workers/container-based-infrastructure/.

Fixes #6078, props tw2113 and netweb.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9262 cdf35c40-ae34-48e0-9cc9-0c9da1808c22