Update 2014-05-08-building-an-ember-app-with-rails-part-2.md

Tests fail on my end due to lack of Ember being defined.

Adding an Ember import to the top of the file.

Note: I'm _super_ new to ES6 modules, so if there is a way to not have to import Ember in the file, I'd love to know what that is.
Merge pull request #187 from thewatts/patch-1

Add Ember import