Upgrade Sequel from 4.12.0

[jszwedko]

Sequel is currently pinned to a pretty old version. I'd like to upgrade it, but this is currently blocked by a change to how Sequel handles validation of serialized fields (changing to validating after rather than before serialization).

I found a Sequel Google Groups thread that discusses this, but it never arrived at a work around for Citygram's use case. I posted a message there to see if anyone has some ideas.

I just wanted to open this issue to track progress in case anyone else makes a go at this. There's no specific need to upgrade just yet, but this particular dependency is pretty stale.