Sequel adapter using Model hooks

Hello!

Because of issue #12 I wrote another Sequel adapter that uses ``Sequel::Model`` hooks. It's called ``sequelhooks`` (I've never been great at naming things). This is for backwards-compatibility on the one hand and on the other hand for faster execution in comparison to, say, a configuration entry which would have to be checked every time a new job is created. ``rspec`` says it works fine.

Minor updates in ``.gemspec``: I upgraded ``rspec`` because it failed to execute with ``2.3.0`` and changed ``sqlite3-ruby`` to ``sqlite3`` because that's its name now.

I would love to see it merged. Maybe you want to release this fix as ``navvy 0.3.3``?

Best regards.