fix(package): update clean-css to version 4.0.9

https://greenkeeper.io/
2017-03-15 10:04:39 +00:00 · 2017-03-15 10:04:39 +00:00 · 76b2c52850
--- a/package.json
+++ b/package.json
@ -8,7 +8,7 @@
  },
  "dependencies": {
    "addons-linter": "0.17.0",
-    "clean-css": "4.0.8",
+    "clean-css": "4.0.9",
    "clean-css-cli": "4.0.8",
    "less": "2.7.2",
    "stylus": "0.54.5",