Use jsx-i18n transform to handle <$_> before React

Summary: I just wrote an npm package called `jsx-i18n` that transforms `<$_>` the same way React currently does, then defers to React's transform for any other JSX transformations. Here I use it.

Test Plan:
Edited `node_modules/jsx-i18n/transforms/react.js` to append `(/*i18n*/` instead of `(` after the tag name for testing, then saw that the built output for passage-ref.jsx looked like

```
return React.DOM.span(null,
    $_(/*i18n*/{lineRange: "?" + EN_DASH + "?"},
```

in the devtools. With React 0.12, the span will use createElement but `$_` will be left alone.

Reviewers: aria

Reviewed By: aria

Differential Revision: http://phabricator.khanacademy.org/D13930