Add automated testing for prod/min builds.

This adds automated testing in Travis for both `ember.prod.js` and `ember.min.js`.

You can run the build tests locally:

* From a console: `rake test[built]`
* From a browser (after running `rackup`): `http://localhost:9292/?skipPackage=container,ember-testing&dist=min&prod=true`

As requested by @stefanpenner.