README

I assume that the bot user is already in some channel.
In my case I had the bot invited to #bot-test in a slack I have admin privileges for.

$ SLACK_API_KEY=xoxb-XXXXXXXXXXXX-XXXXXXXXXXXXXXXXXXXX ruby test-slack.rb

test-slack.rb

require 'rest-client'
require 'faye/websocket'
require 'eventmachine'
require 'json'
require 'yaml'

class RTMSession

  def self.url
    response = RestClient.post(
      "https://slack.com/api/rtm.start",
      token: ENV['SLACK_API_KEY']
    )
    json = JSON.parse(response.body)
    if json.fetch("ok")
      puts "connecting to #{json.fetch('url')}"
      json.fetch("url")
    else
      raise RTMSessionStartFail, json["errors"]
    end
  end

end
class RTMSessionStartFail < StandardError; end

def start(url)
  EM.run {
    ws = Faye::WebSocket::Client.new(url)

    ws.onopen = lambda do |event|
      p [:open, ws.headers]
    end

    ws.onclose = lambda do |close|
      p [:close, close.code, close.reason]
      EM.stop
    end

    ws.onerror = lambda do |error|
      p [:error, error.message]
    end

    ws.onmessage = lambda do |message|
      message_json = JSON.parse message.data
      p [:message, message_json]
    end
  }
end
start(RTMSession.url)