Remove UMAGetSystemVersion() function calling which produced the following
warning on the console for every call:
WARNING: The Gestalt selector gestaltSystemVersion is returning 10.9.4 instead
of 10.10.4. Use NSProcessInfo's operatingSystemVersion property to get correct
system version number.*
Replace UMAGetSystemVersion with wxPlatformInfo::CheckOSVersion() which
internally uses wxGetOSVersion, which uses Gestalt calls not resulting in this
runtime warning.
Closes https://github.com/wxWidgets/wxWidgets/pull/55
658eca2896