Patch to allow 'rake db:migrate:redo' to accept VERSION to target that migration

I find I occasionally type:

rake db:migrate:redo VERSION=20080903190605

and expect it to redo that specific migration, but redo only redoes the most recent migration (or STEPS=x migrations).

I got tired of writing:

rake db:migrate:down VERSION=20080903190605
rake db:migrate:up VERSION=20080903190605

So have patched the rake task to automatically run :down and :up if VERSION is specified for :redo, otherwise default to it's original behavior (:down, :migrate)

There are no tests as I couldn't find where rake tasks were tested. If I've missed it point it out and I'll write tests. Imperical testing suggests it works.