README.OSX: recommend XCode 5 as that's the current version

README.OSX: recommend XCode 5 as that's the current version

Using current "Command Line Tools" with XCode 4 is known to provoke
issues (the build "hangs"), see e.g.

http://stackoverflow.com/questions/18667916/xcrun-lipo-freezes-with-os-x-mavericks-and-xcode-4-x

This fixes https://sourceforge.net/p/openocd/tickets/70/ .

Change-Id: I511b347b9388e0e3f1d136b566915021c4b5fbde
Reported-by: n321203 <n321203@users.sf.net>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2128
Reviewed-by: Xiaofan <xiaofanc@gmail.com>
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>