Improve sourcemaps

This improves source maps, but including a `sourceRoot` value, and generating source maps for `react-intl-with-locales.js`.

**Note:** This should be merged _after_ these two PRs:

- https://github.com/caridy/grunt-bundle-jsnext-lib/pull/7
- https://github.com/caridy/grunt-bundle-jsnext-lib/pull/8
should we use `/` as a prefix here?