Move config folder into src to preserve laravel default path conventions

Main update is moving config folder into src, this way we can preserve laravel path conventions and when you run `php artisan config:publish Anahkiasen/rocketeer-slack` it won't throw an exception, and updated configurationPath in construct, although one have to rewrite config anyway to set package up.

Also updated minor typo in readme, which cost me a time to realise what is wrong with the service provider. :)