Update to reflect recent change in dm-validations.
Update to reflect recent change in dm-validations.

Change in dm-validations was to an `@api private` method, but I don't see how to remove the coupling between dm-types and dm-validations without invasive changes.
ar-dm/ardm-types,engineyard/dm-types,troygnichols/dm-types,datamapper/dm-types,lgierth/dm-types