Assume unknown clang versions are post-Xcode 5

Library/Homebrew/os/mac/xcode.rb

@@ -136,7 +136,7 @@ module OS
           when 41      then "4.5"
           when 42      then "4.6"
           when 50      then "5.0"
-          else "4.6"
+          else "5.0"
           end
         end
       end