Best way to install Capistrano Slack intergration is via Bundler.

Add the following to your Gemfile, then run the bundle command to install the gem direct from the git repository

gem 'capistrano-slack', :git => 'https://github.com/nickvanw/capistrano-slack.git'

Once installed you can use the settings below in your Capistrano deploy.rb to configure Slack.

require 'capistrano/slack'

# required
set :slack_token, "webhook_token" # comes from inbound webhook integration
set :slack_room, "#general"
set :slack_subdomain, "kohactive" # if your subdomain is kohactive.slack.com

# optional
set :slack_application, "Rocketman"
set :slack_username, "Elton John"
set :slack_emoji, ":rocket:"

You can obtain your webhook_token from the integrations section of the team page in Slack.

https://kohactive.slack.com/services/new/incoming-webhook (if your subdomain is kohactive.slack.com)