Use call_user_func for 5.3 compat

Author: jsor

src/React/Promise/Promise.php

@@ -29,7 +29,8 @@ public function then($fulfilledHandler = null, $errorHandler = null, $progressHa
     private function call($callback)
     {
         try {
-            $callback(
+            call_user_func(
+                $callback,
                 array($this->deferred, 'resolve'),
                 array($this->deferred, 'reject'),
                 array($this->deferred, 'progress')