Don't serve far-future expires in development
Don't serve far-future expires in development

This changes the logic for whitehall-frontend so that we proxy assets in
development, but serve far-futures expiry headers in all other environments.

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