bump guzzle requirement

Author: taylorotwell

composer.json

@@ -16,7 +16,7 @@
     },
     "require": {
         "ext-zip": "*",
-        "guzzlehttp/guzzle": "~4.0|~5.0|~6.0",
+        "guzzlehttp/guzzle": "~6.0",
         "symfony/console": "~3.0|~4.0",
         "symfony/filesystem": "~3.0|~4.0",
         "symfony/process": "~3.0|~4.0"