Plugin migration directory should use plugin directory (#14402).

git-svn-id: http://svn.redmine.org/redmine/trunk@14364 e93f8b46-1217-0410-a6f0-8f06a7374b81

lib/redmine/plugin.rb

@@ -432,7 +432,7 @@ module Redmine #:nodoc:
 
     # The directory containing this plugin's migrations (<tt>plugin/db/migrate</tt>)
     def migration_directory
-      File.join(Rails.root, 'plugins', id.to_s, 'db', 'migrate')
+      File.join(directory, 'db', 'migrate')
     end
 
     # Returns the version number of the latest migration for this plugin. Returns