Update GearmanDescriber.php

[ro0NL]

We still see The "Symfony\Component\HttpKernel\Kernel::getRootDir()" method is deprecated since Symfony 4.2, use getProjectDir() instead., coming from GearmanDescriber.php:178

I think the current state is a debug leftover from #196, or im missing it :)

dd(
    method_exists($this->kernel, 'getProjectDir') && false,
    method_exists($this->kernel, 'getProjectDir')
);

^ false
^ true