chore(package): update yargs to version 6.0.0 (#117)

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -39,7 +39,7 @@
     "fs-access": "^1.0.0",
     "object-assign": "^4.1.0",
     "semver": "^5.1.0",
-    "yargs": "^5.0.0"
+    "yargs": "^6.0.0"
   },
   "devDependencies": {
     "chai": "^3.5.0",