Load proper Twig extension for debug functions

The previous extension wasn't right to begin with or was simply outdated. See the current documentation for the [`dump()`](http://twig.sensiolabs.org/doc/functions/dump.html) function which shows how to properly enable this extension starting in Twig 1.5:

```php
$twig = new Twig_Environment($loader, array(
    'debug' => true,
    // ...
));
$twig->addExtension(new Twig_Extension_Debug());
```

This fix should also be patched into 2.2 and master.