new port: security/rubygem-attr_required

This gem provides implementations of attr_required and attr_optional
(like attr_accessor) for Ruby.

WWW: https://github.com/nov/attr_required

PR: 199921
Differential Revision: https://reviews.freebsd.org/D2561
Submitted by: Torsten Zuehlsdorff <ports@toco-domains.de>
Approved by: mat (mentor)