Update CmfBlockExtension initRuntime() typehint

@dbu Without this change, the following error can occur in s Symfony Standard Edition project:

Catchable Fatal Error: Argument 1 passed to Symfony\Cmf\Bundle\BlockBundle\Twig\Extension\CmfBlockExtension::initRuntime() must be an instance of Symfony\Cmf\Bundle\BlockBundle\Twig\Extension\Twig_Environment, instance of Twig_Environment given