Support multiple package versions

Author: Alex Westergaard

composer.json

@@ -9,9 +9,10 @@
         }
     },
     "require": {
-        "guzzlehttp/guzzle": "^6.0"
+        "php": "^7.0 || ^8.0",
+        "guzzlehttp/guzzle": "^6.0 || ^7.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^9.0"
+        "phpunit/phpunit": "^7.0 || ^8.0 || ^9.0"
     }
 }