pass classes mapping when querying sms backend views

Required for upgrade of couchdbkit. Backwards compatible with couchdbkit 0.5.7

```
  File "/home/simon/dev/projects/commcare-hq/corehq/apps/sms/views.py", line 426, in list_backends
    return _list_backends(request, True)
  File "/home/simon/dev/projects/commcare-hq/corehq/apps/sms/views.py", line 393, in _list_backends
    raw_backends += SMSBackend.view("sms/global_backends", include_docs=True).all()
  File "/home/simon/.virtualenvs/commcarehq/src/couchdbkit/couchdbkit/client.py", line 984, in all
    return list(self.iterator())
  File "/home/simon/.virtualenvs/commcarehq/src/couchdbkit/couchdbkit/client.py", line 947, in iterator
    yield wrapper(row)
  File "/home/simon/.virtualenvs/commcarehq/src/couchdbkit/couchdbkit/client.py", line 919, in row_wrapper
    return schema(doc)
  File "/home/simon/.virtualenvs/commcarehq/src/couchdbkit/couchdbkit/schema/util.py", line 37, in wrap
    "This behavior is new starting in 0.6.2.".format(doc_type)
DocTypeError: the document being wrapped has doc type 'GrapevineBackend'. To wrap it anyway, you must explicitly pass in classes={'GrapevineBackend': <document class>} to your view. This behavior is new starting in 0.6.2.```