Django 1.8 deprecation messages fix

Fix of deprecation messages that may appear when you start application server due to django 1.8 deprecations: 

lib/python2.7/site-packages/autocomplete_light/forms.py:449: RemovedInDjango18Warning: Calling modelform_factory without defining 'fields' or 'exclude' explicitly is deprecated
  return django_modelform_factory(model, **kwargs)