Show that we support PHP8 (#238)

Author: Nyholm

composer.json

@@ -29,7 +29,7 @@
         }
     ],
     "require": {
-        "php": "^5.6 || ^7.0",
+        "php": "^5.6 || ^7.0 || ^8.0",
         "psr/cache": "^1.0",
         "symfony/polyfill-php70": "^1.6"
     },