Fix Guzzle5 minimal version

Author: yguedidi

composer.json

@@ -18,7 +18,7 @@
         "php": "^7.1"
     },
     "require-dev": {
-        "guzzlehttp/guzzle": "~5.0",
+        "guzzlehttp/guzzle": "^5.3.1",
         "phpunit/phpunit": "^6.2"
     },
     "suggest": {