databases/rubygem-attr_json: Add rubygem-attr_json 2.1.0

ActiveRecord attributes stored serialized in a json column, super smooth.

Typed and cast like Active Record. Supporting nested models, dirty tracking,
some querying (with postgres jsonb contains), and working smoothy with form
builders.

Use your database as a typed object store via ActiveRecord, in the same models
right next to ordinary ActiveRecord column-backed attributes and associations.
Your json-serialized attr_json attributes use as much of the existing
ActiveRecord architecture as we can.