chore(package): update postcss-cli to version 6.0.0

Closes #1

Author: greenkeeper[bot]

package.json

@@ -30,7 +30,7 @@
     "del-cli": "^0.2.1",
     "http-server": "^0.9.0",
     "npmpub": "^3.1.0",
-    "postcss-cli": "^2.6.0",
+    "postcss-cli": "^6.0.0",
     "postcss-cssnext": "^2.9.0",
     "postcss-import": "^9.0.0",
     "stylelint": "^7.7.0",