Add Xcode 5.1.1 (=Golden master build) clang version

Closes Homebrew/homebrew#28173.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/os/mac.rb

@@ -250,6 +250,7 @@ module OS
       "5.0.1" => { :clang => "5.0", :clang_build => 500 },
       "5.0.2" => { :clang => "5.0", :clang_build => 500 },
       "5.1"   => { :clang => "5.1", :clang_build => 503 },
+      "5.1.1" => { :clang => "5.1", :clang_build => 503 },
     }
 
     def compilers_standard?