Cucumber 0.10.5 was only released in June 2011, but I cannot get the Rake tasks to work with older Cucumbers. Luckily this only affects the development dependencies, so Capybara may in actuality still work with older versions of Cucumber.
Per Jari's comment at #436 (comment)
This way it's far easier to develop against the most recent version of the gem.
Use rspec2's metadata features to skip celerity specs when run in MRI and culerity specs when jruby is not installed.