Fixed: rubytree gem breaks db:migrate:plugins (#4394).

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@3186 e93f8b46-1217-0410-a6f0-8f06a7374b81

vendor/plugins/engines/tasks/engines.rake

@@ -37,6 +37,7 @@ namespace :db do
     desc 'Migrate plugins to current status.'
     task :plugins => :environment do
       Engines.plugins.each do |plugin|
+        next unless plugin.respond_to?(:migration_directory)
         next unless File.exists? plugin.migration_directory
         puts "Migrating plugin #{plugin.name} ..."
         plugin.migrate