Travis CI setup

Added a badge to the readme with build status, and setup the .travis.yml to build appropriately.

This is set to only test on the latest ruby; 2.1.1. Travis has the ability to test multiple versions, but caused a build error with a debugger gem under 1.9.3.

Travis may also require someone with admin permissions on the repository to visit travis-ci.org and turn on builds (steps one and two of http://docs.travis-ci.com/user/getting-started/ - the rest should be taken care of).