fix bug with old version laravel

[enniel]

Bugfix with Laravel 5.1:

 [Symfony\Component\Debug\Exception\FatalErrorException]                                                                                                                                                           
  Class NotificationChannels\Facebook\FacebookServiceProvider contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (Illuminate\Support\ServiceProvider::register) 

[irazasyed]

Thanks!