Prevent unpublished editions from being cached

We currently have a bug where private-frontend, which supports the "preview"
feature in Publisher, will sometimes show the wrong versions of content.

The primary cause was that we were issuing future Expires headers for this
content, which was then cached by Rack::Cache. Unpublished editions should not
be cached.
Rename variable
Merge pull request #113 from alphagov/prevent_unpublished_editions_being_cached

Prevent unpublished editions from being cached