Migrator is unable to handle downgrades from versions it does not know about

[stieg]

This is a scenario that pops up regularly in roll-back situations. Lets define two app: A & B. App A migrates to DB version 3. App B has a new DB migration: 4. App A is initially deployed when app B is rolled out. As expected migrator migrates DB from 3 -> 4. Now say there is a critical bug in app B that requires a roll back. When you roll back to app A the migrator will run and will fail since it does not know what version 4 of the DB is (since this was introduced in app B). This causes failure and cascades into other failures (see #83). Need to figure out how to handle this scenario and fix.