Add Xcode 5.1.1 (GM) clang version

Apple released the golden master build of Xcode 5.1.1. I added its clang version to `os/mac.rb`.

Detailed output of `clang --version`: `Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)`

Fixes the "Homebrew doesn't know what compiler versions ship with your version" warning..