Support RailsAdmin Property class

RailsAdmin recently introduced `Property` class and stopped using old hashy-syntax.
cf. https://github.com/sferik/rails_admin/commit/875c9ba7c18665f768bc48f118c0e9d0c74cb81a
This PR accommodates rails_admin_jcrop to the change, while maintaining backward compatibility.

Refs. sferik/rails_admin#1953