Calculating default for date_created

Prior to this change, the :date_created attribute's default was the
day that the application was last deployed. By using a lambda, the
default for the :date_created will be calculated on initialization.

See https://github.com/projecthydra-labs/active_fedora-registered_attributes/blob/39565686e1ce6235b19ec6a1f0ab949da4ac566e/lib/active_fedora/registered_attributes/attribute.rb#L106-L113

Related to DLTP-267
Merge pull request #157 from jeremyf/date-created-default

Calculating default for date_created