Update guzzle dependency to ^6.3|^7.0

Author: mikemunger

composer.json

@@ -24,7 +24,7 @@
     }
   },
   "require": {
-    "guzzlehttp/guzzle": "^6.3",
+    "guzzlehttp/guzzle": "^6.3|^7.0",
     "league/flysystem": "^1.0",
     "ext-openssl": "*",
     "ext-json": "*"