Laravel 8 Support

[atymic]

Laravel 8 has been released today and we need to update this repo to support it.
See https://laravel.com/docs/8.x/upgrade & laravel-notification-channels/channels#107

[atymic]

Closed in #55

[goranculibrk]

@atymic may I ask how did you install the package in Laravel 8? I'm getting the error for guzzle 6 version, due to the pusher/pusher-push-notification

[atymic]

Looks like it's blocked by this PR:
https://github.com/pusher/push-notifications-php/pull/30/files

[jonathanlloyd]

Looks like it's blocked by this PR:
https://github.com/pusher/push-notifications-php/pull/30/files

Merged. Will release asap.