fix(package): update clean-css to version 4.0.12

https://greenkeeper.io/
2017-04-12 19:08:13 +00:00 · 2017-04-12 19:08:13 +00:00 · c346ea4965
--- a/package.json
+++ b/package.json
@ -8,7 +8,7 @@
  },
  "dependencies": {
    "addons-linter": "0.18.1",
-    "clean-css": "4.0.11",
+    "clean-css": "4.0.12",
    "clean-css-cli": "4.0.11",
    "less": "2.7.2",
    "stylus": "0.54.5",