Fix declaration incompatibility with Slim\View

After this commit the `Slim\Views\Twig::render()` declaration is not compatible with the `Slim\View::render()`

https://github.com/codeguy/Slim/commit/8e3b1fcc303eb9964c08a62404bc415ccb1189a6 

`Strict Standards: Declaration of Slim\Views\Twig::render() should be compatible with Slim\View::render($template, $data = NULL) in /app/vendor/slim/views/Slim/Views/Twig.php on line 46`