Downgrade css-loader dependency for Node 0.10.x compatibility.

Running the build task under Node 0.10.x fails with the following error:

> ERROR in ./~/css-loader!./~/sass-loader!./src/components/widget/style.scss
> Module build failed: ReferenceError: Promise is not defined

See webpack-contrib/css-loader#144 for details.

Node 0.10.x is the latest version available in the Ubuntu package repository, so it's still widely used in dev environments, and should be supported.

package.json

@@ -11,7 +11,7 @@
     "babel-eslint": "^4.1.3",
     "babel-loader": "5.3.2",
     "babel-runtime": "5.8.25",
-		"css-loader": "0.23.0",
+    "css-loader": "0.18.0",
     "eslint": "1.7.3",
     "eslint-plugin-react": "3.6.3",
     "extract-text-webpack-plugin": "^0.9.1",