Fixed get_related_model with hybrid_property.expression

Previously when using a hyprid_property.expression get_related_model 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 belive the original code was just indented wrong accidentially.
Merge pull request #355 from andir/fix_hybrid_property_related

Fixed get_related_model with hybrid_property.expression