Fixes deprecated AR::Migrator constructor used in ar:abort_if_pending_migrations

As of this Rails commit [0], it is no longer legal to pass a string
constructor instead of a list of migration paths. This commit deletes
the deprecated approach and uses the recommended approach, passing the
list of paths to migrate.

[0] https://github.com/rails/rails/commit/685631285f21e2f63b1ecbdb1495f25c97b6bf41
Merge pull request #1510 from fj/topic/remove-ar-deprecations

Fixes deprecated AR::Migrator constructor used in ar:abort_if_pending_migrations