Change guzzle version to ~6.0

composer.json

@@ -14,7 +14,7 @@
     "require": {
         "php": ">=5.4",
         "league/plates": "~3.1",
-        "guzzlehttp/guzzle": "~5.3",
+        "guzzlehttp/guzzle": "~6.0",
         "league/commonmark": "^0.13",
         "symfony/console": "~3.0",
         "symfony/finder": "~3.0",