Fix deploy button?

[crcastle]

Hey @JedWatson I'm a Developer Advocate and I'd love to get this Heroku Button deploy working again. I ran into similar or the same issues as described in #6 and #7. Is getting the button working something you're interested in? If so, would you accept a PR to get it working and update this repo with Keystone v4?

I haven't looked into it in much detail yet, but any pitfalls I should be aware of? Thanks!