Bugfixes for down migrations, and a rollback method

Tried installing this gem on a project I'm working on today. After running the database migrations, as per my usual practice, I tried rolling them back. This failed due two a couple of small bugs in the `down` methods for the migrations.

This pull request fixes those bugs. It also adds a `::rollback` class method (aliased as `::down`) to the `Songkick::OAuth2::Schema` class, to mirror the `::migrate` class method.