chore(package): update yargs to version 6.0.0

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -40,7 +40,7 @@
     "globby": "^6.0.0",
     "posthtml": "^0.9.0",
     "posthtml-load-plugins": "^0.11.2",
-    "yargs": "^5.0.0",
+    "yargs": "^6.0.0",
     "path-exists": "^3.0.0"
   },
   "devDependencies": {