Change priority of DisableCSRFExtension to make it compatible with Symfony 3.2

Hi.

I was testing an app with Symfony 3.2 Beta and encountered the following issue like @romainneutron pointed out in [symfony/symfony#20332](https://github.com/symfony/symfony/issues/20332). I'd suggest to change the priority of the extension in order to avoid having this issue once more people are updating their apps to Symfony 3.2.