Changed migrations specs to require that migration_stub calls exist.

Before, using migration_stub could reference a global model.  The goal
of migration_stub was to prevent loading global models so that we can
ensure that at the least tests enforced that rule.  Now, migration_stub
enforces the rule.

This exposed many places where migration_stub was being used incorrectly
or where the migration itself incorrectly referenced global models.

In addition, 2 migrations weren't testing anything except pure
ActiveMigration code, which isn't worth doing, so they were removed.

@jrafanie Please review.
Good point...I should have it raise on nil.  Otherwise you get a cryptic `undefined method `blah` for NilClass in the spec.  That cryptic error was what I was using to find all the bad stubs :)