Adapt ember-moment to htmlbars new helpers signature

In htmlbars helper methods have a different signature. This PR defines one function or another based on the presence of `Ember.HTMLBars`.

Also fixes some annoying deprecation warnings in apps using htmlbars.
This is better than the interleaved stuff, but even better would be to leave this file mostly untouched but move it into `addon/helpers/handlebars/moment.js` and make a new file `addon/helpers/htmlbars/moment.js`.