Fixed an update_index bug when using multiple connections

I would get this warning `/usr/local/lib/python2.7/dist-packages/haystack/query.py:340: UserWarning: The model <class 'reclamos.models.Empresa'> is not registered for search.` and the update_index command wouldn't remove objects deleted from the model when using `--remove`, because the management command wasn't taking into account the backend specified with the `--using=` parameter.