Skip tests for symfony 2.3

Twig 2 and symfony 2.3 are not actually compatible.

Tests/Twig/Extension/SonataAdminExtensionTest.php

@@ -89,6 +89,10 @@ class SonataAdminExtensionTest extends \PHPUnit_Framework_TestCase
 
     public function setUp()
     {
+        // NEXT_MAJOR: remove this block when dropping symfony < 2.7 support
+        if (!class_exists('Symfony\Bridge\Twig\Extension\AssetExtension')) {
+            $this->markTestSkipped();
+        }
         date_default_timezone_set('Europe/London');
 
         $container = $this->getMock('Symfony\Component\DependencyInjection\ContainerInterface');