Fix "Declaration of Slim\Views\Twig::render() should be compatible..."

After codeguy/Slim@8e3b1fcc303eb9964c08a62404bc415ccb1189a6, the signature of `Slim\View::render()` changed, so the Twig view should change too.

For reference this is the error that it's throwing right now:

    PHP Strict Standards:  Declaration of Slim\Views\Twig::render() should be compatible with Slim\View::render($template, $data = NULL) in .../Slim/Views/Twig.php on line 46