Run tests headlessly on Travis CI

This is a spike to get the test suite from jquery-ujs up and running on Travis CI so we can get a glimpse of contributions builds on the Pull Requests. 

I managed to get a build running on my here [https://travis-ci.org/lucasmazza/jquery-ujs/builds/13340457](here). There is a single test broken related to a 403 xhr that seems to be mangled by PhantomJS (both status code and error message are missing) that I want to take a closer look later, but this enough for anyone to comment about the setup and everything else.

I had to update the QUnit assets since the [runner script](https://github.com/jquery/qunit/blob/master/addons/phantomjs/runner.js) addon seems to target APIs that our version didn't supported yet. All the specs look green in the browser.