unicorn-rails is a simple gem that sets the default server for rack (and rails) to unicorn.
unicorn-rails overrides the
Rack::Handler.default method to return
Rack::Handler::Unicorn which will cause rack (and
rails) to use unicorn by default.
gem install unicorn-rails
git clone https://github.com/samuelkadolph/unicorn-rails cd unicorn-rails && rake install
gem "unicorn-rails" to your Gemfile and then run
rails s and it will use unicorn by default.