Bump govuk_content_models to 4.13.0

The methods that were originally added to the Artefact model with
meta-programming have since been moved into the model as part of
4.13.0 of govuk_content_models. For more context, see:
https://github.com/alphagov/govuk_content_models/pull/62

The attr_accessor has been left in place as it's a Rummageable method
added as part of the module and not something we want to expose on the
model for the time being.

The context of the attr_accessor can be found in
7734351324a80b89b46d00e8903fc6b194c8b858
Use the archived? helper method instead of accessing the field

This helper method already exists and is being used across the app. Be
consistent and use it instead of differences between this and the
state field equality check.
Merge pull request #74 from alphagov/bump-content-models-to-4.13.0

Bump govuk_content_models to 4.13.0