chore(package): update stylelint to version 6.0.0 (#129)

http://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -152,7 +152,7 @@
     "sass-loader": "^3.1.2",
     "strip-markdown": "^0.3.1",
     "style-loader": "^0.13.0",
-    "stylelint": "^5.0.0",
+    "stylelint": "^6.0.0",
     "stylelint-config-standard": "^5.0.0",
     "webpack": "^1.12.11",
     "whatwg-fetch": "^0.11.0"