Save known users to robot brain

Been having issues with the pagerduty script not recognizing or remembering users, and I think it has to do with `hubot-slack` not appending user data into the brain like so:
https://github.com/github/hubot/blob/master/src/adapters/campfire.coffee#L56-L67

Looking into my redis database, it's looking like the "users" key is totally empty, which I don't think is expected a adapters.

Thoughts? Any rationale for the users hash not being used, or was it just an oversight that hadn't caused issues yet?

Thanks!