Can't enable keepAlive on ssl (wss) server
Server

```
$MyServer = new MyServer();

$app = new Ratchet\Http\HttpServer(
    new Ratchet\WebSocket\WsServer(
        $MyServer
    )
);

$loop = React\EventLoop\Factory::create();

$secure_websockets = new React\Socket\Server('0.0.0.0:6190', $loop);
$secure_websockets = new React\Socket\SecureServer($secure_websockets, $loop, [
    'local_cert' => '/path/to/cert.pem',
    'local_pk' => '/path/to/privkey.pem',
    'verify_peer' => false
]);

$secure_websockets_server = new Ratchet\Server\IoServer($app, $secure_websockets, $loop);

$app->enableKeepAlive($app->loop, 30);

$secure_websockets_server->run();
```
The server runs fine without `$app->enableKeepAlive($app->loop, 30);`.

With it, I get
`PHP Fatal error:  Uncaught Error: Call to undefined method Ratchet\Http\HttpServer::enableKeepAlive() in /path/to/server-ssl.php:27`

Thanks in advance for any pointers.