Update gulp to 4.0

Not at all urgent, but noting that the gulp version we're on is out of date, and has a few dependencies with vulnerabilities (that are harmless to this repo) that GitHub flags. I marked those vulnerabilities as irrelevant to this repo.

Updating to gulp 4 is non-trivial, as it has breaking API changes - from my brief testing, it will likely require a rewrite (or re-paste/adapt) of `gulpfile.js`.