Revert "diagnostic: make using outdated development tools fatal"

This reverts commit a124680b189f50ebeb550845e3c0efd34db66247.

Will need to be smarter than this, since people can't force Travis to
update. I'm losing count of the amount of times Travis has forced a change
of plans around Homebrew this year.

Closes https://github.com/Homebrew/brew/issues/1096.

Library/Homebrew/extend/os/mac/diagnostic.rb

@@ -14,10 +14,15 @@ module Homebrew
       end
 
       def fatal_development_tools_checks
-        %w[
-          check_xcode_up_to_date
-          check_clt_up_to_date
-        ]
+        if MacOS.prerelease?
+          %w[
+            check_xcode_up_to_date
+            check_clt_up_to_date
+          ]
+        else
+          %w[
+          ]
+        end
       end
 
       def check_for_unsupported_macos