Upgrade to Hubot 3

[smashwilson]

There's a new, shiny, all-JavaScript Hubot release in the works. I've offered to upgrade pushbot early as a guinea pig to test it out and flush out any bugs.

• npm install --save hubot@next should do the ⬆️ trick
• If I hit warnings related to bad peerDependencies, there's a new pattern I can follow to tidy them up

/cc @technicalpickles so you can see when I get around to this 👋