A bit wary of `rake` in the Gemfile because any upgrades will result in the annoying "You have already activated x but your Gemfile requires y". Also because we'll need to `bundle exec rake` all the time (which, granted, is less annoying with a simple alias).

The alternative is to leave rake out, and consider it a system dependency (similar to foreman/postgres/etc). Probably a terrible idea if we do anything non-standard with rake, but figured it's worth considering. What do you think?