Add PHP 8 support (#4)

Author: mtvbrianking

composer.json

@@ -18,13 +18,13 @@
         }
     ],
     "require": {
-        "php": "^7.2.5",
+        "php": "^7.2.5|^8.0",
         "illuminate/support": "^6.0|^7.0|^8.0",
         "simshaun/recurr": "^4.0"
     },
     "require-dev": {
         "orchestra/testbench": "^6.0",
-        "phpunit/phpunit": "^9.3"
+        "phpunit/phpunit": "^8.4|^9.3"
     },
     "autoload": {
         "psr-4": {