Require dependencies before using them.

This change `require`s the main files of the gems that `middleman-ember` depends on before referencing constants defined in those files, so gems are always loaded in the correct order.

The `require` sequence correctly reflects the fact that `ember-source` is a required dependency, but `ember-data-source` is an optional one.

Thank you very much for packaging ember.js for use with middleman! Please consider merging this change, to make the integration a tiny bit easier.