bot.rb

require 'slack-ruby-client'

Slack.configure do |conf|
  conf.token = 'TOKEN'
end

client = Slack::RealTime::Client.new

client.on :hello do
  puts 'connected!'
end

client.on :message do |data|
  case data['text']
  when 'にゃーん' then
    client.message channel: data['channel'], text:'Λ__Λ'
  end
end

client.start!

Gemfile

source 'https://rubygems.org'

gem 'slack-ruby-client'
gem 'eventmachine'
gem 'faye-websocket'