Avoid warnings about deprecated Ember.required
Avoid warnings about deprecated Ember.required

As of Ember 1.12 a deprecation warning is emitted to the console when
using `Ember.required`:

```
DEPRECATION: Ember.required is deprecated as its behavior is inconsistent and unreliable.
        at Object.required (http://genesis.rbthree.exa-online.de/static-assets/vendor.js:23929:22)
        at http://genesis.rbthree.exa-online.de/static-assets/vendor.js:79332:17
        at http://genesis.rbthree.exa-online.de/static-assets/vendor.js:80572:2
```

So, remove any usage of `Ember.required` since it hasn't really worked properly.

See https://github.com/emberjs/ember.js/pull/10668

zendesk/ember-resource,eXa-online/ember-resource,zendesk/ember-resource,eXa-online/ember-resource