Update CmfBlockHelper.php to work with CmfBlockExtension and sonata_block_render

The CmfBlockExtension extends the Sonata\BlockBundle\Twig\Extension\BlockExtension which defines `sonata_block_render` in its (extended) getFunctions() method.  The `sonata_block_render` simple Twig function will call the CmfBlockHelper render() method, so it should be more like the Sonata\BlockBundle\Templating\Helper\BlockHelper render() method.