Silence furder Django 1.8 warnings about ContentType imports. Fix get_model in handlers.

mayan/apps/metadata/handlers.py

@@ -2,6 +2,8 @@ from __future__ import unicode_literals
 
 import logging
 
+from django.db.models import get_model
+
 from .tasks import task_add_required_metadata_type, task_remove_metadata_type
 
 logger = logging.getLogger(__name__)
@@ -36,7 +38,7 @@ def post_post_document_type_change_metadata(sender, instance, **kwargs):
     for metadata in instance.metadata.all():
         metadata.delete(enforce_required=False)
 
-    DocumentMetadata = sender.get_model('DocumentMetadata')
+    DocumentMetadata = get_model('metadata', 'DocumentMetadata')
 
     # Add new document type metadata types to document
     for document_type_metadata_type in instance.document_type.metadata.filter(required=True):