Updated to work with sequel 3.x

Hi,

I recently needed this plugin while working with an existing AR-based codebase. I was using the latest sequel 3.18.x and noticed that sequel_polymorphic was no longer compatible. So I've updated it and it now passes all test using that version of sequel. 

If you're no longer maintaining or able to maintain this then perhaps I should post a note about this on the sequel mailing list?

Regards,

Saimon