OK, it's two weird old tricks.
First, use their bundle caching feature. It's one line added to your .travis.yml
file, and significantly speeds up test runs where you haven't changed your gem bundle - which is most of the time for most people.
See more here: http://blog.travis-ci.com/2013-12-05-speed-up-your-builds-cache-your-dependencies/
Second, do some experimentation with your Ruby garbage collection settings to speed up your local test suite runs. Examples from our development environment: