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

[yaffs-website] / web / core / modules / rdf / tests / rdf_test_namespaces / rdf_test_namespaces.info.yml

name: 'RDF module namespaces test'
type: module
description: 'Test namespace declaration.'
package: Testing
version: VERSION
core: 8.x
dependencies:
  - drupal:rdf