Updated all rake tasks to work with Sequel 3.42's API

Heya,

I updated all rake tasks to work with Sequel 3.4.2 - the API for migrations looks like it has changed noticeably, and some of the methods that were public related to latest version and setting migration version are now private - I've marked where I've violated Sequel's visibility with FIXME comments.

Please let me know what you think.

- Max