Fix "Declaration should be compatible..."

After codeguy/Slim@8e3b1fc, the signature of `Slim\View::render()`
changed, so the Twig and Smarty views should change too.

For reference this is the error that it's throwing right now (Twig):

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