Calling a Specific Gem Version In The Console

Today I was trying to generate a Rails 3.1.0 project (for testing purposes). I simply called rails new rollback which generated a rails 3.2.11 project and proceeded to change the gemfile version to 3.1.0. This resulted in numerous application errors. The simple solution was to generate the project with the Rails version 3.1.0. To do this call the gem with the version number as such:

$ rails _3.1.0_ new rollback

You also must already have this version of the gem installed on your system for it to work.