can't run cucumber scenarios due to test-unit version issue on Rails 2.3.5, Ruby 1.9.1

Question

I've been trying to follow along in the RSpec book, (I'm new to all of this) and I have what appears to be some kind of versioning issue. If I try and run some simple scenarios, I get this error:

can't activate test-unit (= 1.2.3, runtime) for [], already activated test-unit-2.0.7 for [] (Gem::LoadError) /Users/jeffdeville/.rvm/rubies/ruby-1.9.1-p378/lib/ruby/site_ruby/1.9.1/rubygems.rb:230:in activate' /Users/jeffdeville/.rvm/rubies/ruby-1.9.1-p378/lib/ruby/site_ruby/1.9.1/rubygems.rb:1056:ingem' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/rspec-1.3.0/lib/spec/interop/test.rb:4:in <top (required)>' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/polyglot-0.3.1/lib/polyglot.rb:64:inrequire' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/polyglot-0.3.1/lib/polyglot.rb:64:in require' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:158:inrequire' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/rspec-1.3.0/lib/spec/test/unit.rb:1:in <top (required)>' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/polyglot-0.3.1/lib/polyglot.rb:64:inrequire' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/polyglot-0.3.1/lib/polyglot.rb:64:in require' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:158:inrequire' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/rspec-rails-1.3.2/lib/spec/rails.rb:13:in <top (required)>' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/polyglot-0.3.1/lib/polyglot.rb:64:inrequire' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/polyglot-0.3.1/lib/polyglot.rb:64:in require' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:158:inrequire' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/cucumber-rails-0.3.0/lib/cucumber/rails/rspec.rb:15:in rescue in <top (required)>' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/cucumber-rails-0.3.0/lib/cucumber/rails/rspec.rb:3:in' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/polyglot-0.3.1/lib/polyglot.rb:64:in require' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/polyglot-0.3.1/lib/polyglot.rb:64:inrequire' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:158:in require' /Users/jeffdeville/code/showtime/Features/support/env.rb:11:in' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/polyglot-0.3.1/lib/polyglot.rb:64:in require' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/polyglot-0.3.1/lib/polyglot.rb:64:inrequire' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/cucumber-0.6.4/lib/cucumber/rb_support/rb_language.rb:124:in load_code_file' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/cucumber-0.6.4/lib/cucumber/step_mother.rb:85:inload_code_file' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/cucumber-0.6.4/lib/cucumber/step_mother.rb:77:in block in load_code_files' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/cucumber-0.6.4/lib/cucumber/step_mother.rb:76:ineach' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/cucumber-0.6.4/lib/cucumber/step_mother.rb:76:in load_code_files' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/cucumber-0.6.4/lib/cucumber/cli/main.rb:48:inexecute!' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/cucumber-0.6.4/lib/cucumber/cli/main.rb:20:in execute' /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378/gems/cucumber-0.6.4/bin/cucumber:8:in' script/cucumber:9:in load' script/cucumber:9:in'

however, uninstalling 2.0.7 yields the error:

Missing these required gems: test-unit = 2.0.7

You're running: ruby 1.9.1.378 at /Users/jeffdeville/.rvm/rubies/ruby-1.9.1-p378/bin/ruby rubygems 1.3.6 at /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378, /Users/jeffdeville/.rvm/gems/ruby-1.9.1-p378@global

Run rake gems:install to install the missing gems.

Sorry this is probably something easy, but I just don't know ruby or rails well enough yet.