WampServerInterface not found
Fatal error: Uncaught Error: Interface 'Ratchet\Wamp\WampServerInterface' not found in pusher.php on line 7

Pusher Code:
 namespace app;
  use Ratchet\ConnectionInterface;
  use Ratchet\Wamp\WampServerInterface;

  class Pusher implements WampServerInterface {
      protected $subscribedTopics=array();
      public function onSubscribe(ConnectionInterface $conn, $topic) {
          $this->subscribedTopics[$topic->getId()]=$topic;
      }

      public function onUnSubscribe(ConnectionInterface $conn, $topic) {
      }

      public function onOpen(ConnectionInterface $conn) {
      }

      public function onClose(ConnectionInterface $conn) {
      }

      public function onCall(ConnectionInterface $conn, $id, $topic, array $params) {
        // In this application if clients send data it's because the user hacked around in console
        $conn->callError($id, $topic, 'You are not allowed to make calls')->close();
      }

      public function onPublish(ConnectionInterface $conn, $topic, $event, array $exclude, array $eligible) {
        echo $topic." | ".$event["name"];    
      }

      public function onError(ConnectionInterface $conn, \Exception $e) {
        echo "An error has occurred: {$e->getMessage()}\n";
        $conn->close();
      }
  }

########################################################

Composer Json:
{
    "name": "Ratchet",
    "description": "Testing Ratchet Websocket",
    "type": "php",
    "license": "proprietary",
    "minimum-stability": "alpha",
    "autoload": {
        "psr-4": {
            "app\\":"app/"
        }
    },
    "require": {
        "cboden/ratchet": "^0.4.3"
    }
}

Any help will be welcome :)