Merge pull request DATA-DOG#51 from tamago-db/fix-doctrine-dbal-json

json_encode the diff (doctrine/dbal 2.6 BC break)

src/DataDog/AuditBundle/EventSubscriber/AuditSubscriber.php

@@ -381,7 +381,7 @@ protected function diff(EntityManager $em, $entity, array $ch)
                 ];
             }
         }
-        return $diff;
+        return json_encode($diff);
     }
 
     protected function assoc(EntityManager $em, $association = null)