Do Heroku Cedar stack apps require thin and eventmachine in Gemfile?
gem 'thin'
gem 'eventmahine'
What about versions? On other stacks they were installed automatically?
Yes, you need to specify any gem that you want to use on Cedar stack unlike on Bamboo which had preinstalled gems.
Go with whatever version your application needs,
来源：https://stackoverflow.com/questions/8362303/heroku-cedar-stack-thin-and-eventmachine