Replace kernel.root_dir by kernel.project_dir

kernel.root_dir and Kernel::getRootDir() are deprecated since Symfony 3.3.
See https://symfony.com/blog/new-in-symfony-3-3-a-simpler-way-to-get-the-project-root-directory and https://github.com/symfony/symfony/blob/3.3/UPGRADE-3.3.md#httpkernel for more information.