Laravel: Support `model` config from 5.2+

[DFurnes]

Laravel 5.2 has the model field (which specifies which Eloquent model is used for authed users) in a different place. We need to add support for both (or just force Laravel 5.2 for future releases).

 // config/auth.php for Laravel 5.1
 'model' => App\Models\User::class,

 // config/auth.php for Laravel 5.2
 'providers' => [
     'users' => [
         'driver' => 'eloquent',
         'model' => App\Models\User::class,
     ],
 ],