gem 'redis-rails'
gem 'sidekiq'
gem 'sinatra', github: 'sinatra/sinatra', require: false
gem 'devise-async'

Devise::Async.backend = :sidekiq

Mailer.delay.signup_email(@user, @password)

2016-10-26T19:02:09.473Z 17854 TID-ww0i8 WARN: ArgumentError: undefined class/module Mailer