Bump govuk_content_models to 6.0.1

This allows us to add content from Departments and Policy without
causing erroneous validation errors due to the way suffixes are used to
differentiate clashing URLs.
Merge pull request #108 from alphagov/content-models-601

Bump govuk_content_models to 6.0.1
Actually bump govuk_content_models

The previous commit had some erroneous bumps that we didn't want. This
reverts those, and bumps the gems we actually want.