fix. sprint's story don't reflect to move to top or bottom, when enable auto-refresh.

platform:  # supported: 2.0.4, 2.1.6, 2.2.3
backlogs:  # supported: 0.9.36
ruby:  # supported: 1.9.3, 1.8.7

fix. sprint's story don't reflect to move to top or bottom, when enable auto-refresh.

@see https://github.com/backlogs/redmine_backlogs/blob/b292e36fb86fa9c408352c8903244a31f148c329/app/models/rb_story.rb#L241