Bump api-adapters and content_models

This bumps gds-api-adapters and govuk_content_models. The change to
api-adapters includes support for the fields `public_timestamp` and 
`latest_change_note` in the Panopticon registerer. 

The change to govuk_content_models adds new methods to the `Edition`
model to access the major changes for an edition, the latest change note
for a major change, and the updated_at timestamp of the last major
change.
Remove old Edition enhancements

As this behaviour is now included in the `Edition` model in
govuk_content_models, this switches the `PublishedEditionPresenter` to
use the new `public_updated_at` method. This also adds additional unit
test coverage.
Send correct change note to the Publishing API

This changes the `PublishedEditionPresenter` so that the
`latest_change_note` method is used to populate the `change_note` key,
rather than using the `change_note` attribute.

This means that the change note provided will always be from the most
recent _major_ change, rather than the most recent published edition.
Send the latest change fields to Panopticon

This changes the `RegisterableEdition` class so that the
`public_timestamp` and `latest_change_note` fields are included when
an edition is registered with Panopticon.
Merge pull request #309 from alphagov/send-public-timestamp-and-change-note-to-rummager

Send information about an edition's latest change to Panopticon