Fix remove_validations_for for AS 4.1

This is a slightly reworked version of the @fcheung's pull
request #584, using the same method across ActiveSupport
versions.  I added a definition for `set_callbacks`, which is
present in AS 4.1 but not in earlier versions.  This can be
discarded once older versions of AS are no longer supported.
Fix remove_validations_for fix for AS 3.x/4.x
Merge pull request #587 from cpmurphy/fixes/keys_active_support_4_1

Fix remove_validations_for for AS 4.1