Fixed get_related_model with hybrid_property.expression

Previously when using a `hybrid_property.expression`, the `get_related_model()` function would throw an exception like:

      [...]
      File "..python3.4/site-packages/flask_restless/views.py", line 1056, in get
        return self._search()
      File "..python3.4/site-packages/flask_restless/views.py", line 999, in _search
        relations = frozenset(get_relations(self.model))
      File "..python3.4/site-packages/flask_restless/helpers.py", line 114, in get_relations
        return [k for k in dir(model)
      File "..python3.4/site-packages/flask_restless/helpers.py", line 116, in <listcomp>
        and get_related_model(model, k)]
      File "..python3.4/site-packages/flask_restless/helpers.py", line 129, in get_related_model
        if isinstance(attr, AssociationProxy):
    UnboundLocalError: local variable 'attr' referenced before assignment

I believe the original code was just indented wrong accidentally.