Update ValidationPass to work with Symfony 2.5

This should fix #1498
instead of detecting the version of the kernel, I would rather use feature detect to identify the version of FrameworkBundle (it is possible to have a different version for FrameworkBundle and HttpKernel when installing standalone components):

```php

if ($container->hasDefinition('validator.builder')) {
    // Symfony 2.5+
    $container->getDefinition('validator.builder')
        ->addMethodCall('addXmlMapping', array($validationFile));

    return;
}

// Old code goes here