gc_enable is not called if running with HHVM

Since HHVM doesn't support garbage collector (for obvious reasons), the following patch make Ratchet works with HHVM as well.

src/Ratchet/Server/IoServer.php

@@ -33,7 +33,10 @@ class IoServer {
      * @param \React\EventLoop\LoopInterface|null $loop     The React looper to run the Ratchet application off of
      */
     public function __construct(MessageComponentInterface $app, ServerInterface $socket, LoopInterface $loop = null) {
-        gc_enable();
+        if (false === strpos(PHP_VERSION, "hiphop")) {
+            gc_enable();
+        }
+
         set_time_limit(0);
         ob_implicit_flush();
 
@@ -128,4 +131,4 @@ class IoServer {
     public function handleError(\Exception $e, $conn) {
         $this->app->onError($conn->decor, $e);
     }
-}
+}