Avoid deprecated notice when using symfony/config > 4.2

[odolbeau]

See: https://github.com/symfony/symfony/blob/master/UPGRADE-4.2.md#config

[matthieumota]

I purpose a more concise syntax

$rootNode = \method_exists(TreeBuilder::class, 'getRootNode') ? $treeBuilder->getRootNode() : $treeBuilder->root('stof_doctrine_extensions');

[odolbeau]

@matthieumota: updated!

[garethbraid89]

Changed a couple of variable names to make sure they match older ones

[odolbeau]

I didn't pay attention to that, thanks @garethbraid89!

[franmomu]

This bundle supports "php": ">=5.3.2" so TreeBuilder::class cannot be used because was introduced in PHP 5.5.0. So I would use doctrine/DoctrineBundle#853 approach.

[matthieumota]

Okay so @odolbeau just need to pass treebuilder instance rather than FQN instruction.

[stof]

Actually closing because #388 also fixed it.