diagnostic: make using outdated dev tools fatal

A smarter form of 559cea7fa9.
Travis users can't force Travis to update 10.11 to Xcode 8, so this was murdering
builds left, right & centre.

Fixes https://github.com/Homebrew/brew/issues/1096 whilst still retaining the
point of the original commit. Also offers developers an opt-out so if we need
to test something on 10.11 with Xcode 7.x we can, etc.

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

@@ -14,13 +14,13 @@ module Homebrew
       end
 
       def fatal_development_tools_checks
-        if MacOS.prerelease?
+        if ENV["TRAVIS"] || ARGV.homebrew_developer?
           %w[
-            check_xcode_up_to_date
-            check_clt_up_to_date
           ]
         else
           %w[
+            check_xcode_up_to_date
+            check_clt_up_to_date
           ]
         end
       end