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.
I suggest putting a helper at the top of this file:

```js
var callHelper;
if (Ember.HTMLBars) {
  callHelper = function(helper, args) {
    return helper(args);
  }
} else {
  callHelper = function(helper, args) {
    return helper.apply(null, args);
  }
}
```
Then the tests don't branch with `if`. Much easier to maintain.

I'm not sure what `FAKE_HANDLEBARS_CONTEXT` is doing. Passing it as the second argument to HTMLBars means it will be considered the hash options a user passed `{{moment foo=bar}}`. I don't believe that is the intent, maybe only the Handlebars code needs this?