Avoid timestamp migration collisions, please review [#4412]

I've come across a use case for creating potentially many db
migrations in an application at once. This causes a problem with
timestamped migrations, which will all receive the same time stamp
(assuming the script creates them within the same second.)

I've made a small patch to address it, please review. Thanks!

https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/4412