Added transaction commiting after the SQL update

It appears that the transaction would need to be managed externally
in order to actually update the DB.  Now do it at the end of the
bulk_update call.

bulk_update/helper.py

@@ -139,3 +139,4 @@ def bulk_update(objs, meta=None, update_fields=None, exclude_fields=None,
             del values, pks
 
             connection.cursor().execute(sql, parameters)
+            transaction.commit_unless_managed(using=using)