public function extendTransportManager(TransportManager $manager)
 {
     $manager->extend('sendgrid', function () {
         $config = $this->app['config']->get('services.sendgrid', array());
         $client = new HttpClient(Arr::get($config, 'guzzle', []));
         if (Arr::get($config, 'version') === 'v3') {
             return new SendgridV3Transport($client, $config['api_key']);
         }
         return new SendgridTransport($client, $config['api_key']);
     });
 }