diagnostic: Add CircleCI to Env check

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

@@ -53,8 +53,8 @@ module Homebrew
         return unless MacOS::Xcode.installed?
         return unless MacOS::Xcode.outdated?
 
-        # Travis CI images are going to end up outdated so don't complain.
-        return if ENV["TRAVIS"]
+        # CI images are going to end up outdated so don't complain.
+        return if ENV["TRAVIS"] || ENV["CIRCLECI"]
 
         message = <<-EOS.undent
           Your Xcode (#{MacOS::Xcode.version}) is outdated.