revert "use DI's Extension class directly" to ensure Symfony 2.1 compatibility

This reverts commit 1c27de4.

Author: craue

DependencyInjection/CraueConfigExtension.php

@@ -4,8 +4,8 @@
 
 use Symfony\Component\Config\FileLocator;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
-use Symfony\Component\DependencyInjection\Extension\Extension;
 use Symfony\Component\DependencyInjection\Loader\XmlFileLoader;
+use Symfony\Component\HttpKernel\DependencyInjection\Extension;
 
 /**
  * Registration of the extension via DI.