Use Ember.Handlebars in helpers guide

This is pretty nit picky, but if JSHint is configured out of the box with your build tools to accept Ember as a global, this save you from having JShint throw "Handlebars in undefined" errors if you copy the example directly from the guide.
Merge pull request #1515 from southpolesteve/patch-1

Use Ember.Handlebars in helpers guide