Allow the use of pusher-php-server 4.0 (#209)

Author: ttomdewit

composer.json

@@ -32,7 +32,7 @@
         "illuminate/http": "5.7.* || 5.8.*",
         "illuminate/routing": "5.7.* || 5.8.*",
         "illuminate/support": "5.7.* || 5.8.*",
-        "pusher/pusher-php-server": "~3.0",
+        "pusher/pusher-php-server": "~3.0 || ~4.0",
         "symfony/http-kernel": "~4.0",
         "symfony/psr-http-message-bridge": "^1.1"
     },