Show that we support PHP8 (#238)

Author: Nyholm

composer.json

@@ -19,7 +19,7 @@
         }
     ],
     "require":           {
-        "php":                      "^5.6 || ^7.0",
+        "php": "^5.6 || ^7.0 || ^8.0",
         "psr/cache":                "^1.0",
         "psr/simple-cache":         "^1.0",
         "cache/hierarchical-cache": "^0.4"