chore: add Laravel 8 support

Author: mxschll

composer.json

@@ -13,13 +13,13 @@
     ],
     "require": {
         "php": ">=7.2",
-        "guzzlehttp/guzzle": "^6.5",
-        "illuminate/notifications": "~5.5 || ~6.0 || ~7.0",
-        "illuminate/support": "~5.5 || ~6.0 || ~7.0"
+        "guzzlehttp/guzzle": "^6.5 || ^7.0",
+        "illuminate/notifications": "~5.5 || ~6.0 || ~7.0 || ^8.0",
+        "illuminate/support": "~5.5 || ~6.0 || ~7.0 || ^8.0"
     },
     "require-dev": {
         "mockery/mockery": "^1.3",
-        "phpunit/phpunit": "^8.0"
+        "phpunit/phpunit": "^8.0 || ^9.0"
     },
     "autoload": {
         "psr-4": {