Use pangeo-bot to update dependencies

[jhamman]

@charlesbluca setup @pangeo-bot at the #pangeo2019 sprint. This bot mimics the functionality of @henchbot, creating pull requests to this repo every time there is a new version of repo2docker or BinderHub available. This seems to work well (#69).

This issue is meant to:

1. coordinate the final implementation of the bot, notably the Heroku deployment we were working on at the end of the sprint.
2. brainstorm other things we want this bot do (for example, it may be nice if the bot automatically opened a new PR to prod after each merge to staging)
3. thank @charlesbluca for his help on this

[jhamman]

see #107 for a partial solution.