Setup Travis.ci for testing and build status

I added in the following Rubies to test against:

rvm:
  - 1.9.2
  - 1.9.1
  - ruby-head
  - rbx
  - ree

You'll have to login to travis-ci, OAuth to github and click the on switch for this repo since you're the maintainer. Click the test hook on github and then the first build should trigger.