Add Guzzle 7.x to allowed versions.

Author: BusterNeece

composer.json

@@ -16,7 +16,7 @@
     "minimum-stability": "RC",
     "require": {
         "php": ">=5.6.0 <8.0",
-        "guzzlehttp/guzzle": "^6.3.0",
+        "guzzlehttp/guzzle": "^6.3.0|^7.0.0",
         "codeception/lib-innerbrowser": "^1.0",
         "codeception/codeception": "^4.0"
     },