chore(package): update postcss to version 6.0.1

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -47,7 +47,7 @@
     "property"
   ],
   "dependencies": {
-    "postcss": "^5.2.10"
+    "postcss": "^6.0.1"
   },
   "devDependencies": {
     "ava": "*",