Move react-source gem out of project root

Now it requires a build step. At least the confusing lib directory is gone though.

One day we'll stop copy+pasting the npm/gem build tasks around and abstract some of this...
aha!

> Feel free to use dynamic code in here. When your gem is built, Rubygems will run that code and create a static representation. This means it’s fine to pull your gem’s version or other shared details out of your library itself. Do not, however, use other libraries or dependencies.
> http://yehudakatz.com/2010/04/02/using-gemspecs-as-intended/