Update api_key.py

I added the parentheses to the `db.session.rollback` line to call the method, which will now properly roll back any changes made to the database if an error occurs.

powerdnsadmin/models/api_key.py

@@ -83,7 +83,7 @@ class ApiKey(db.Model):
         except Exception as e:
           msg_str = 'Update of apikey failed. Error: {0}'
           current_app.logger.error(msg_str.format(e))
-            db.session.rollback
+          db.session.rollback()  # fixed line
           raise e
 
     def get_hashed_password(self, plain_text_password=None):