chore(package): update standard to version 8.4.0

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -34,7 +34,7 @@
     "babel-preset-es2015": "^6.3.13",
     "chokidar": "^1.2.0",
     "mocha": "^3.0.2",
-    "standard": "^7.1.2"
+    "standard": "^8.4.0"
   },
   "dependencies": {
     "icss-replace-symbols": "^1.0.2",