How to switch between different version of gem installed?

Question

I have three version of rack installed on local machine (rack (1.4.1, 1.3.6, 1.3.5)). For some gem (such as Cucumber), it requires a lower version of <

Answer 1

You can specify a version in gemfile of your project

gem "rack",  "1.3.5"

Pointed by matt:

To use gem specified in Gemfile:

bundle exec cucumber