Add travis ci integration

I've added integration with the Travis Continuous Integration server, which is useful testing against various versions of Ruby.

Hopefully you find it useful.  If you decide to accept the pull, you should:

1. Sign up for a Travis CI account at https://travis-ci.org (you can use your Github account)
2. Add Travis settings in your Github Account.  You can do this by clicking on Settings-> Service Hooks->Travis.  Enter your Travis username and token and hit Save.
3. Edit the README to change the travis build status icon to point to your repository (currently it's pointing to my joe1chen branch).