Remove Rails 2.3 support

This PR removes Rails 2.3 support. Rails 2.3 and its immediate successor, Rails 3.0, are no longer supported by the Rails core team. Given that maintaining compatibility with Rails 2.3 makes it impossible to take advantage of useful patterns like `ActiveSupport::Concern`, I think the Octopus community gets the most benefit by removing Rails 2.3 support going forward.