chore(package): update postcss to version 5.2.3 (#153)

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -17,7 +17,7 @@
     "nprogress": "0.2.0",
     "onmount": "1.3.0",
     "pjax": "0.2.4",
-    "postcss": "5.2.2",
+    "postcss": "5.2.3",
     "pug": "^2.0.0-beta6",
     "stylus": "0.54.5",
     "uglifyify": "3.0.3",