Show that we support PHP8 (#238)

Author: Nyholm

composer.json

@@ -23,7 +23,7 @@
         }
     ],
     "require": {
-        "php": "^5.6 || ^7.0",
+        "php": "^5.6 || ^7.0 || ^8.0",
         "psr/cache": "^1.0",
         "cache/adapter-common": "^1.0"
     },