If you have a Rails 2.x app and hitting this, there is a simple fix while you wait to upgrade to Rails 3.

Edit your environment.rb file and inbetween the require ‘boot’ line and the Rails::Initializer line, insert the following Gem::VERSION if statement:

# Bootstrap the Rails environment, frameworks, and default configuration
require File.join(File.dirname(__FILE__), 'boot')

if Gem::VERSION >= "1.3.6"
  module Rails
    class GemDependency
      def requirement
        (super == Gem::Requirement.default) ? nil : super
      end
    end
  end
end

Rails::Initializer.run do |config|
# ....

It’s a hack, it might break with later and later releases of RubyGems. But it allowed me to get a 2.3.x Rails app running on a fairly current rubygems.