Update reference to retrieve the jstransform visitors.

The commit facebook/react@8dbc530d1cee38289bea0e3e61f4ff89f7340e47 changed the exported function name for getting the visitors in `fbtransform` from `getVisitorsList` to `getAllVisitors`.

The change breaks `src/Packager.js` on our end, and this pull request fixes it by updating the method call.