Fix applications with custom user models first install

[antonkomarev]

Fixes #300

This PR removes hardcoded User model value from config file, otherwise there is issues on first install and you are not able to publish configuration file.

Illuminate\Foundation\ComposerScripts::postAutoloadDump
php artisan package:discover

In MessengerServiceProvider.php line 102:

Class 'App\User' not found

By default package will use auth.providers.users.model value to resolve User model, what covers most of the use cases.

[cmgmyr]

Thanks!