The old Laravel 4 system of overriding config based on development environment is no longer functional, it was removed in Laravel 5 because it was incredibly slow. If you want to change the values of certain config options, you need to use environment variables which are stored in the .env file. See config/app.php for some examples
Alternatively, if you want to use subdirectories for additional config: I've never done it with config, but for language files this worked for me in the past: Lang::get('path/to/dir/file.key.subkey');