Fixes for current active_model_serializers version with Rails

Hey there:

You might be able to shed some light on why, but before I explicitly defined `#as_json`, `#to_json`, and `#serializable_hash` as delegates to the versioned instance, they were using the default implementations and yielding a hash representation which looked like this:

```
{ options: [ ... ], object: { ... }
```

This was when using the gem under Rails 3.2 with the latest version of active_model_serializers (0.8.1).

Anyway, if you think this is worth pulling, go for it :)