use external script packages

This change upgrades a bunch of packages, and transitions to using external hubot script packages, where possible. See deprecation warning [here](https://github.com/github/hubot-scripts) for more info.

I would be lying if I said the impetus for this change wasn't largely driven by wanting to have https://github.com/hubot-scripts/hubot-google-images/pull/1 included :wink: