Building Gemfile with Multiple Rails versions

Rails 4.0.0 is ready and you want to test your application with it.

Add the following to your Gemfile:

gem "rails", ENV["RAILS_VERSION"] || "3.2.12"

To rspec your application with new rails do the following:

export RAILS_VERSION=4.0.0; bundle update; bundle exec rspec spec

Update:

See the following post: Testing Against Multiple Rails Versions