Merge pull request #1261 from jitsi/clean-css-module-update

Stick to clean-css version 3.x.

package.json

     "babel-preset-es2015": "^6.18.0",
     "babel-preset-react": "^6.16.0",
     "babel-preset-stage-1": "^6.16.0",
-    "clean-css": "*",
+    "clean-css": "^3.0.0",
     "css-loader": "*",
     "eslint": "^3.13.1",
     "eslint-plugin-jsdoc": "*",