Target a specific version of handlebars_assets in building an Ember app part 1

In the latest version of `handlebars_assets` Handlebars was updated to RC4, breaking compatibility with the current version of Ember.js. 

This pull fixes that by targeting a specific version of `handlebars_assets` that still uses RC3.
Merge pull request #46 from rpwll/patch-1

Target a specific version of handlebars_assets in Building an Ember App