brew-update: Guard non-directory files in Taps

Skip any entries returned by `Dir["Library/Taps/*"].each` that are not
directories.

Library/Homebrew/cmd/update.rb

@@ -19,6 +19,8 @@ module Homebrew extend self
 
     new_files = []
     Dir["Library/Taps/*"].each do |tapd|
+      next unless File.directory?(tapd)
+
       cd tapd do
         begin
           updater = Updater.new