[BREAKING ISSUE] Js is no longer in the global namespace of js_of_ocaml.

[benbot]

This breaks reason-reactify.

A version constraint should be set up in package.json or the references to the Js module should be updated.

Unfortunately, I'm pretty new to ocaml/reasonml so I might not be able to get a PR put together in a reasonable time. I'll work on it, but I thought an issue should be opened anyway.