Fix class loading order for deprecated classes

The deprecated classes (Drupal and Symfony) now extend Drupal8 and
Symfony2, so those need to be loaded first.

Otherwise you get errors like:

```
Fatal error: Class 'Boris\Loader\Provider\Drupal8' not found in /Users/Scott/bin/boris-loader/src/Boris/Loader/Provider/Drupal.php on line 9
```

```
Fatal error: Class 'Boris\Loader\Provider\Symfony2' not found in /Users/Scott/bin/boris-loader/src/Boris/Loader/Provider/Symfony.php on line 9
```