See here for the reasoning: http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/
Quick version: these changes remove our use of sudo so that we can move to the container-based builds. This yields quicker builds and less reliance on the old infrastructure. It looks to be ~30%-50% quicker, though it's hard to tell because there's so much variance.
Since we can't use sudo, we now have to handle packages in the build-matrix rather than as part of the build step.
The second commit is pretty much unrelated, I just noticed that there was a gap in testing here.