Don't run upgrades unless this is an incremental serialization.

If you're running serializer on 4.0.x, this code would try to apply all
the upgrade steps up to 4.2.y before writing out the object.

Instead - limit to the current RT version.
Noticed when running rt-serializer on 4.0 and incremetal upgrades ran.

$ref is a hashref, not an object, so you need to access {Domain} not
call the Domain method.
Merge branch '4.2/avoid-migration-upgrades' into 4.2-trunk