Update composer.json

[wanghanlin]

support Laravel 5.5 and 5.6

close #23
close #19

[jcrawford]

It would appear this does not quite fix things for 5.5 or 5.6. The reason is that the pusher-php-server at version 2.6.4 will not work with 5.5. prior to version 3.0 the Pusher class was in the global namespace. As of Laravel 5.5 they started looking for Puaher\Pusher which is only available with pusher-php-server 3.0

I can install 3.0 manually but this library requires 2.6.4 so i cannot seem to get around a composer install

Your requirements could not be resolved to an installable set of packages.

Problem 1
- laravel-notification-channels/pusher-push-notifications 1.0.4 requires pusher/pusher-php-server 2.6.* -> satisfiable by pusher/pusher-php-server[2.6.1, 2.6.2, 2.6.3, 2.6.4, v2.6.0] but these conflict with your requirements or minimum-stability.

using 2.6.4 I get the following errors.

In BroadcastManager.php line 213:
Class 'Pusher\Pusher' not found

[wanghanlin]

@jcrawford Yes, I realised that already and submitted #27 and used new pusher API, but it's not been merged