config_environments_development.rb

module MyApplication
  class Application < Rails::Application
    config.logger = Logger.new($stdout)
    config.middleware.insert_before(::Rack::Lock, ::Rack::LiveReload)
  end
end

Guardfile

group :livereload do
  guard 'livereload' do
    watch(things)
  end
end

Procfile

rails: rails s
guard: guard -g livereload -i