class ZendeskAppsSupport::AppVersion

At any point in time, we support up to four versions:

* deprecated -- we will still serve apps targeting the deprecated version,
                but newly created or updated apps CANNOT target it
* sunsetting -- we will soon be removing support for this version;
                updated apps SHOULD target the current version, no new apps allowed
* current    -- we will serve apps targeting the current version;
                newly created or updated apps SHOULD target it
* future     -- we will serve apps targeting the future version;
                newly created or updates apps MAY target it, but it
                may change without notice