Update restrictions for guzzlehttp/guzzle package.

Closes #3397677

composer.json

@@ -6,6 +6,6 @@
   "minimum-stability": "dev",
   "require": {
     "php": ">=5.5.0",
-    "guzzlehttp/guzzle": "~6.0"
+    "guzzlehttp/guzzle": "~6.0||~7.5.1"
   }
 }