refs https://github.com/TryGhost/Toolbox/issues/280
- In response to 'Accept-Version' header in the request headers, Ghost will always respond with a content-version header indicating the version of the Ghost install that is responding. This should signal to the client the content version that is bein g served
- This is a bare bones implementation and more logic with edge cases where `content-version` is served with a version value of "best format API could respond with" will be added later.