Merge pull request #44 from ADmad/php-8

Allow installing on PHP 8.

Author: shadowhand

composer.json

@@ -16,7 +16,7 @@
         }
     ],
     "require": {
-        "php": "^7.1",
+        "php": "^7.1 || ^8.0",
         "psr/http-factory": "^1.0",
         "phpunit/phpunit": "^6.5 || ^7.0 || ^8.0 || ^9.0"
     },