chore(package): update postcss to version 5.2.5

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -63,7 +63,7 @@
     ]
   },
   "dependencies": {
-    "postcss": "^5.1.1"
+    "postcss": "^5.2.5"
   },
   "devDependencies": {
     "ava": "^0.16.0",