plant should handle models that no longer exist

[spulec]

If a model is in a dump, but no longer in the current environment, plant currently blows up.

  File "../lib/python2.7/site-packages/django/core/management/commands/loaddata.py", line 190, in handle
    for obj in objects:
  File "../lib/python2.7/site-packages/django/core/serializers/json.py", line 47, in Deserializer
    raise DeserializationError(e)
DeserializationError: Invalid model identifier: 'app.model'

It should instead log a warning and move onto the next model.