fix(package): update clean-css to version 3.4.24

https://greenkeeper.io/
2017-01-20 07:45:00 +00:00 · 2017-01-20 07:45:00 +00:00 · 8ef6a78f85
--- a/package.json
+++ b/package.json
@ -8,7 +8,7 @@
  },
  "dependencies": {
    "addons-linter": "0.15.15",
-    "clean-css": "3.4.23",
+    "clean-css": "3.4.24",
    "less": "2.7.2",
    "stylus": "0.54.5",
    "uglify-js": "2.7.5"