Make environment js files erb to enable reading values from ENV.

As discussed tonight at the Ember.js for Rails Devs workshop at Dockyard offices:
Reading config values from environment instead of hardcoding keys in your JS files is a best practice and important for environments like heroku (also possible with on non-heroku environments using the Figaro gem). In order to do this, the environment js files need to be .erb.

Not sure if the example I placed in production.js.erb is sufficient.