chore(package): update postcss to version 6.0.1

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -8,7 +8,7 @@
   },
   "dependencies": {
     "icss-replace-symbols": "1.0.2",
-    "postcss": "5.2.9",
+    "postcss": "6.0.1",
     "postcss-modules-values": "1.2.2",
     "postcss-modules-extract-imports": "1.0.1",
     "postcss-modules-local-by-default": "1.1.1",