fix(package): update postcss to version 6.0.21

Closes #48

Author: greenkeeper[bot]

package.json

@@ -54,7 +54,7 @@
     "fs-extra": "5.0.0",
     "loader-utils": "1.1.0",
     "parse5": "3.0.3",
-    "postcss": "6.0.17",
+    "postcss": "6.0.21",
     "postcss-load-config": "1.2.0"
   },
   "devDependencies": {