I’ve extracted and improved upon common code from slack-gamebot and slack-mathbot and made slack-ruby-bot. It’s probably the easiest way to make a Slack bot in Ruby today.
Here’s a minimal example.
gem 'slack-ruby-bot' to your Gemfile and party on. Unlike Lita it’s not trying to be anything other than a Slack bot boilerplate. Source code on Github.
Update: Integration with Rails
If you’re trying to integrate Slack with Rails, check out Writing a Slack Bot running on Rails (with React).