Fixes error 500 on documents#show

Fixes Error on documents#show (Redmine 2.3):

An ActionView::Template::Error occurred in documents#show:

undefined method 'link_to_attachment_with_preview' for #<#<Class:0xc3ada70>:0xc2fbdc0>
  plugins/redmine_lightbox/app/views/attachments/_links.html.erb:3:in '_plugins_redmine_lightbox_app_views_attachments__links_html_erb___1061661976_102111850'