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 dep.

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