2.x holding branch

[jrochkind]

We're going to merge work for a potential 2.x release into this branch. It's somewhat experimental, so we're keeping it out of master.

I'm going to do PR's into this branch, and try to label them all v2 for reference.

Goals of this release are getting closer to Rails attributes API/use, and if possible reducing code in attr_json. For some of this, jsonb_accessor serves as a reference

• Make rails_attribute default true, and/or remove the configuration and make it always on
• Can we make Rails dirty tracking work via Rails attributes, and not need our separate dirty tracking implementation at all?
• Make AttrJson::Model use the ActiveModel::Attributes API (which didn't exist in Rails 5.0 when we started this; so this would go along with making the gem no longer support Rails 5.0). Ideally a fairly light-weight shim on top of Attributes API.