Updated Drupal to 8.6. This goes with the following updates because it's possible...

[yaffs-website] / web / core / modules / book / tests / modules / book_test_views / book_test_views.info.yml

name: 'Book test views'
type: module
description: 'Provides default views for views book tests.'
package: Testing
version: VERSION
core: 8.x
dependencies:
 - drupal:book
 - drupal:views