Refactor Travis config

This change will download the correct version of PhantomJS for each build instead of downloading all of them.

I'll also look into getting SlimerJS working with this new format. Though I'm not sure if we should wait until SlimerJS has headless support or if we could use [this](http://about.travis-ci.org/docs/user/gui-and-headless-browsers/).

Any input would be greatly appreciated.