xcode: update regex

Ensures we capture Clang's fourth-digit when it exists. This seems to be on pre-release
versions of OS X only, but is the cause of the misdetection of CLT up-to-date status
on 10.11 several weeks ago.

For full explanation, see Homebrew/homebrew#42261.

Closes Homebrew/homebrew#42261.

Library/Homebrew/os/mac/xcode.rb

@@ -178,7 +178,7 @@ module OS
         else
           version = `/usr/bin/clang --version`
         end
-        version = version[%r{clang-(\d+\.\d+\.\d+)}, 1] || "0"
+        version = version[%r{clang-(\d+\.\d+\.\d+(\.\d+)?)}, 1] || "0"
         version < latest_version
       end