rack-rewrite with Rails 3.2.3 on Heroku
===

I'm trying to set up rack-rewrite for a Rails 3.2.3 on Heroku cedar stack and everywhere I look it says to add something like:

    config.middleware.insert_before(Rack::Lock, Rack::Rewrite) do
      # rewrite rules
    end

But trying to run this on Heroku gives me the error:

    `assert_index': No such middleware to insert before: Rack::Lock (RuntimeError)

and indeed running "heroku rake middleware"
doesn't show Rack::Lock on the list while in development Rack::Lock is there.

First question is why Rack::Lock is not present on Heroku, is that correct?
Second question, if not before Rack::Lock, where should I insert Rack::Rewrite?

Thanks!