css sourcemaps no longer working after installing ember-paper

Noticed that after installing ember-paper, the app's built CSS file no longer includes the `/*# sourceMappingURL=my-app.css.map */` line.

This occurs even if the `@import "ember-paper";` line is removed from `app.scss`.

Here is a repo that demonstrates the issue: https://github.com/bantic/try-css-sourcemaps

If you pull that down and `ember build`, you should see that the `dist/assets/try-css-sourcemaps.css` file does not have that sourceMapping url. Removing `ember-paper` from the package.json and rebuilding will cause the sourceMapping url to reappear.