use Travis CI's newer, more reliable infrastructure

[Faster Builds with Container-Based Infrastructure and Docker][1] says
to add `sudo: false` to try new infrastructure.  Claims tests will start
running in seconds instead of minutes and have fewer infrastructure
related failures.

[1]:
http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/
Merge pull request #55 from nnutter/travis-ci

use Travis CI's newer, more reliable infrastructure