Replace id by pk in Indexable

Author: ei-grad

elasticutils/contrib/django/__init__.py

@@ -307,4 +307,4 @@ def get_indexable(cls):
 
         """
         model = cls.get_model()
-        return model.objects.order_by('id').values_list('id', flat=True)
+        return model.objects.order_by('pk').values_list('pk', flat=True)