fixed Twig provider

src/Silex/Provider/TwigServiceProvider.php

@@ -45,7 +45,7 @@ class TwigServiceProvider implements ServiceProviderInterface
                 $twig->addExtension(new \Twig_Extension_Debug());
             }
 
-            if (class_exists('Symfony\Bridge\Twig\TwigEngine')) {
+            if (class_exists('Symfony\Bridge\Twig\Extension\RoutingExtension')) {
                 if (isset($app['url_generator'])) {
                     $twig->addExtension(new TwigRoutingExtension($app['url_generator']));
                 }