Have CI automatically test migration paths

We can have a build that runs a script that:

• Resets db/schema.rb and db/fixtures to the last release (based on the VERSION file and the v[VERSION] tag)
• Runs rake db:setup and rake db:seed_fu (or a subset thereof) to set up the database with some "old" data
• Runs rake db:migrate to migrate to new data
• Verifies that all of this happens without errors

We could do the same thing on EE to simulate a CE->EE upgrade. This way we should be able to prevent stuff like https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/290 from happening.

cc @ayufan @marin @dzaporozhets @zj

Thoughts?