Use new, supported APIs in OS X 10.10 and iOS 8.0 to get the OS version.

Gestalt is deprecated so we can't use it long term. At the same time,
the new API is cross platform, so we'll no longer have to parse strings
in -[UIDevice systemVersion] either.

Change-Id: Ic81797174c1a3d50b47b9b209205a6a506cc75ef
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>