CoffeeScript 1.7's compiler must be registered explicitly

CoffeeScript 1.7.0 brought this change:
```
When requiring CoffeeScript files in Node you must now explicitly register the compiler. This can be done with require 'coffee-script/register' or CoffeeScript.register(). Also for configuration such as Mocha's, use coffee-script/register.
```

Without this change, coffee files get compiled using the version of coffee-script that comes with Grunt, which is currently at "~1.3.3".