Change the healthcheck endpoint for Content API
Change the healthcheck endpoint for Content API

The endpoint `/vat-rates.json` no longer exists, so we need to change
the healthcheck endpoint for now until we turn this app off.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet