Support newer Guzzle versions (#10)

Update composer.json

Author: marktopper

composer.json

@@ -6,7 +6,7 @@
     "type": "package",
     "require": {
         "laravel/framework": ">=5.1",
-        "guzzlehttp/guzzle": "~6.0"
+        "guzzlehttp/guzzle": ">=6.0"
     },
     "autoload": {
         "psr-4": {