Added Laravel 8.x support (#55)

Author: henrydennis-sonin

composer.json

@@ -14,8 +14,8 @@
     "require": {
         "php": ">=7.1.3",
         "guzzlehttp/guzzle": "^6.2 || ^7.0",
-        "illuminate/notifications": "~5.6 || ~6.0 || ~7.0",
-        "illuminate/support": "~5.6 || ~6.0 || ~7.0",
+        "illuminate/notifications": "~5.6 || ~6.0 || ~7.0 || ~8.0",
+        "illuminate/support": "~5.6 || ~6.0 || ~7.0 || ~8.0",
         "kreait/laravel-firebase": "^1.3 || ^2.1",
         "spatie/enum": "^2.3 || ^3.0"
     },