Use Travis Container-Based Infrastructure

This may help speed up our builds on Travis according to
http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/
Add Travis Caching

Not sure about this one, I'll monitor and see if this works.
Test Travis build speed
Remove DIY Travis caching

Travis has introduced it's own caching strategy now...
Remove bin/wad
Merge pull request #550 from G5/faster-travis

Travis CI - Container-Based Infrastructure