Allow guzzle v7

Author: gmponos

composer.json

@@ -22,7 +22,7 @@
         "ext-curl": "*",
         "ext-json": "*",
         "ext-mbstring": "*",
-        "guzzlehttp/guzzle": "^6.2"
+        "guzzlehttp/guzzle": "^6.2 || ^7.0"
     },
     "require-dev": {
         "phpunit/phpunit": "^4.8",