Update to 'relationship' terminology
Improve error message

`fixture` is an instance of `Object`.

Before:
```
the id property must be defined for fixture [Object object]
```

After:
```
the id property must be defined for fixture {"foo": "bar"}
```
Use `toString` if environment doesn't support `JSON.stringify`
Remove unused variables and helper functions
upgrade ember
utilize the new Computed#readOnly
prefer public DI api on the application instance when possible.
bump ember test-version
record becomes clean if it reverts to original attributes and relationships