What's the proper way to downgrade a gem?

I recently installed the Rails3.1-Devise-Rspec-Cucumber Starter App with the Gemfile listed below. This created a Gemfile.lock file (relevant snippet below) that includes factory_girl 2.0.0rc3. Unfortunately, this version of FactoryGirl is apparently completely busted.

What's the proper way to force my bundle to use factory_girl 2.0.0.rc1 instead of 2.0.0rc3?

Gemfile:

source 'http://rubygems.org'
gem 'rails', '3.1.0.rc4'
gem 'mysql2'
gem 'sass-rails', "~> 3.1.0.rc"
gem 'coffee-script'
gem 'uglifier'
gem 'jquery-rails'
gem "therubyracer", ">= 0.8.2"
gem "rspec-rails", ">= 2.6.1", :group => [:development, :test]
gem "f开发者_如何学JAVAactory_girl_rails", ">= 1.1.beta1", :group => :test
gem "cucumber-rails", ">= 1.0.0", :group => :test
gem "capybara", ">= 1.0.0", :group => :test
gem "database_cleaner", ">= 0.6.7", :group => :test
gem "launchy", ">= 0.4.0", :group => :test
gem "devise", ">= 1.3.4"

Relevant snippet of Gemfile.lock

factory_girl (2.0.0.rc3)
factory_girl_rails (1.1.rc1)
  factory_girl (~> 2.0.0.rc)
  railties (>= 3.0.0)

gem "factory_girl", "2.0.0.rc1", :group => :test

in your gem file, and then run

bundle update factory_girl