Docs aren't being published

[mythmon]

In CircleCI our docs-publish job is failing with this error:

$ gh-pages --dotfiles --message "[skip ci] Docs updates" --dist docs/_build/html

The "file" argument must be of type string. Received type undefined
Exited with code 1

Here is an example CI job: https://circleci.com/gh/mozilla/normandy/12787

This job only runs on master, so bors hasn't noticed it. This problem seems to have started recently. I'd guess it is because we don't actually pin a version of the gh-pages tool, and install it via npm install gh-pages@2. We should probably move this to package.json and manage it like all the other Node dependencies.

[glasserc]

This build failure seems to have stopped as of gh-pages 2.1.1: https://circleci.com/gh/mozilla/normandy/13389. See tschaub/gh-pages#312. But I still think the idea of explicitly versioning gh-pages is valuable, so I'll do that.