Specify current Xcode to aid issue-diagnosis

I saw a ticket with just doctor output and no `brew --config`. This helps.

Library/Homebrew/cmd/doctor.rb

@@ -208,7 +208,7 @@ def check_for_latest_xcode
     else "4.3"
   end
   if MacOS.xcode_version < latest_xcode then <<-EOS.undent
-    Your Xcode version is outdated
+    You have Xcode-#{MacOS.xcode_version}, which is outdated.
     Please install Xcode #{latest_xcode}.
     EOS
   end