Update composer.json to be able to run with php8 (#625)

* Update composer.json

php8

* Update composer.json

Author: sietzekeuning

composer.json

@@ -21,7 +21,7 @@
         }
     ],
     "require": {
-        "php": "^7.2",
+        "php": "^7.2|^8.0",
         "omnipay/common": "^3",
         "php-http/discovery": "^1.12",
         "php-http/guzzle7-adapter": "^0.1"