appraise 'rails-4' do
  gem 'rack', '< 2'
  gem 'rails', '= 4.2.3'
end

appraise 'rails-5.0' do
  gem 'rails', '~> 5.0.0'
end

appraise 'rails-5.1' do
  gem 'rails', '~> 5.1'
end