Coq 8.4pl3 (version upgrade, style nit, + `head` update)

If y'all would prefer I remove [the `s/ENV.j1/ENV.deparallelize/` commit][1] and/or create a separate pull-request for updating Coq's `head`, I'd be happy to oblige.

By the way, we could use [the GitHub mirror][2] instead of their [gforge repo][3] for the `head` URL. Let me know if one's preferred over the other.

[1]: e4a638d384f6ef64f3c52ebf0ca32fa0c466a71a
[2]: https://github.com/coq/coq.git
[3]: git://scm.gforge.inria.fr/coq/coq.git