setup development deps to work with JRuby, pin to rails 3.x

Small change before I start attempting anything a bit more serious. I updated the gemspec development deps so that they will work with JRuby. I wanted to be able to run the specs in MRI and JRuby. Had to swap RDiscount for Kramdown and check the platform for the correct sqlite gem to load.

In addition, _auditable_ does not work with the latest AcitveRecord 4.x, since the `attr_accesible` has been moved to its own gem. I pinned the ActiveSupport and ActiveRecord to `~> 3.0` for now.

    RuntimeError: `attr_accessible` is extracted out of Rails into a gem. Please use new recommended protection     model for params(strong_parameters) or add `protected_attributes` to your Gemfile to use old one.
    attr_accessible at /home/zinger/.rbenv/versions/jruby-1.7.4/lib/ruby/gems/shared/gems/activemodel-4.0.0/lib/active_model/deprecated_mass_assignment_security.rb:14