Added PHP 8 support

Author: mnocon

composer.json

@@ -15,7 +15,7 @@
             "MarekNocon\\ComposerCheckout\\Tests\\": "tests/"}
     },
     "require": {
-        "php": "^7.1",
+        "php": "^7.1 || ^8.0",
         "composer-plugin-api": "^2.0",
         "ext-json": "*"
     },