Fix order of array_first arguments

As per https://laravel.com/docs/5.3/upgrade (under "Arrays")

src/Notification/NotificationSyncer.php

@@ -89,7 +89,7 @@ public function sync(Blueprint\BlueprintInterface $blueprint, array $users)
                 continue;
             }
 
-            $existing = $toDelete->first(function ($i, $notification) use ($user) {
+            $existing = $toDelete->first(function ($notification, $i) use ($user) {
                 return $notification->user_id === $user->id;
             });