Fix expected JSON for ActiveModelAdapter.

The current implementation of DS.ActiveModelAdapter needs the relationship keys to be defined in a different format. See reference https://github.com/emberjs/data/blob/v1.0.0-beta.8/packages/activemodel-adapter/lib/system/active_model_serializer.js#L86

```js
"posts": [{
  "id": 1,
  "title": "foo",
  "body": "foo",
  "author_id": <author id> 
}],
"authors": [{
  "id": 1,
  "first_name": "foo",
  "last_name": "foo",
  "post_ids": [<post ids>] 
}]
```