Add Travis CI

Alright, here it comes... #104

I have set up a travis.yml file for configuration:

- It tests against the latest rails 3 and rails 4 versions
- It tests against ruby versions: 1.9.2, 1.9.3, 2.0.0, 2.1.0
- It spins up the Faye rack app before running the tests

And as expected I ran into some issues while running the tests:

- In order to make the tests run in 1.9.2, I had to add a `'minitest', '< 5.0.0'` as a development dependency.
- `String.first` somehow does not work for Rails 4.0.3. So I replace it with `String[0]` in `message_test.rb`

So, now you can enable CI by logging into travis-ci.org with your github credentials and switch on the sync repo in the settings. Then you can grab the fancy badge and paste it into the readme. :tada: