Fix the JSON representation of the artefacts index.

Calling `respond_with` with more than one resource only really makes
sense in contexts where it's expected to create a redirect (eg in a
create action), and therefore calls `polymorphic_url`.  In this case it
was causing the json view of the index to render `@tag_collection`
instead of the artefacts.
Exclude the actions field from the artefacts index.

They're not used in the index view, and omitting them improves
performance somewhat.
Bump content models to 17.1.1

This is to pick up the fix for Artefact#as_json with missing tags
(alphagov/govuk_content_models#213).
Merge pull request #194 from alphagov/fix_json_index

Fix the JSON representation of the artefacts index.
Bump govuk_content_models version to 18.0.0

Adds new Artefact kinds for drug_safety_updates and medical_safety_alerts
Merge pull request #195 from alphagov/feature/bump-content-models

Bump govuk_content_models version to 18.0.0