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>

README.OSX

@@ -3,8 +3,8 @@ Building OpenOCD for OSX
 
 There are a few prerequisites you will need first:
 
-- Xcode 4 (install from the AppStore)
-- Command Line Tools (install from Xcode 4 -> Preferences -> Downloads)
+- Xcode 5 (install from the AppStore)
+- Command Line Tools (install from Xcode 5 -> Preferences -> Downloads)
 - MacPorts (http://www.macports.org/install.php)
   or
 - Homebrew (http://mxcl.github.io/homebrew/)