Avoid run-time warnings for OS X version checks under 10.10+.

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

include/wx/osx/core/private.h

@@ -54,8 +54,6 @@
 // wxBase part of it).
 #include <Carbon/Carbon.h>
 
-WXDLLIMPEXP_BASE long UMAGetSystemVersion() ;
-
 void WXDLLIMPEXP_CORE wxMacStringToPascal( const wxString&from , unsigned char * to );
 wxString WXDLLIMPEXP_CORE wxMacMakeStringFromPascal( const unsigned char * from );