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