Add PHP 8 Support (#32)

* Add PHP 8 support

Author: joelrose

composer.json

@@ -3,7 +3,7 @@
     "description": "SumUp eCom SDK for PHP",
     "type": "library",
     "license": "proprietary",
-    "version": "1.0.2",
+    "version": "1.1.0",
     "keywords": ["sumup", "sdk", "payment processing", "ecommerce", "payment", "checkout"],
     "homepage": "https://developer.sumup.com",
     "authors": [
@@ -14,7 +14,7 @@
         }
     ],
     "require": {
-        "php": "^5.6|^7.0"
+        "php": "^5.6|^7.0|^8.0"
     },
     "suggest": {
       "guzzlehttp/guzzle": "Allows for implementation of the Guzzle HTTP client"