gemfile <<-RB
  gem 'rack'
  gem 'redis', '~>3.2.1'
RB

gemfile <<-RB
  gem 'rack'
  gem 'redis', '3.0.0' # oldest supported version
RB

gemfile <<-RB
  gem 'rack'
  gem 'redis', '2.2.2' # newest unsupported version
RB
