Load proper Twig extension for debug functions

The previous extension wasn't right to begin with or was simply outdated when trying to use the dump function, resulting in an error like: `The function "dump" does not exist in twig file ...`

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());
```

You can then properly use `{{ dump(var) }}` in your templates.

This fix should also be patched into 2.2 and master.