Add php 8 support.

Author: Rodrigo Aguilera

composer.json

@@ -9,7 +9,7 @@
         }
     ],
     "require": {
-        "php": "^7.1",
+        "php": ">=7.1 ",
         "nyholm/psr7": "^1.3",
         "php-http/httplug": "^2.0",
         "psr/http-client": "^1.0",