I actually want to revisit this again... what happens if we do change the API in JSXTransformer? Then we're out of luck, right? (there's been a discussion about getting the AST out, and we're adding sourcemaps support which might cause issues).

I also think if the versions aren't tied together we should make it clear that not specifying `react-source` here is *ok* and you'll just get whatever version of React we declare in `react-rails`. But if you do specify a version for `react-source` you can override easily without dropping files into a specific place.

Does that sound reasonable?