Use Travis containers for faster tests
Use Travis containers for faster tests

See:
http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/

janusnic/clearance,pedrosmmoreira/clearance,benlivermore/clearance,thoughtbot/clearance,naveed-ahmad/clearance,janusnic/clearance,naveed-ahmad/clearance,janusnic/clearance,thoughtbot/clearance,zernie/clearance,thoughtbot/clearance,zernie/clearance,benlivermore/clearance,naveed-ahmad/clearance,benlivermore/clearance,zernie/clearance,pedrosmmoreira/clearance,pedrosmmoreira/clearance