When upgrading to Lumen 5.0.4, you should update your bootstrap/app.php file's creation of the Lumen application class to the following:
$app = new Laravel\Lumen\Application(
realpath(__DIR__.'/../')
);
Note: This is not a required change; however, it should prevent some bugs when using the Artisan CLI and PHP's built-in web server.
Lumen 5.0 is the initial release of the Lumen framework, and is based on the Laravel 5.x series of PHP components.