fix translation-missing in rails 4

In Rails 4, when the TranslationHelper#t fails to find a translation, it tries to render a `span.translation_missing` using the `content_tag` helper.

see: https://github.com/rails/rails/blob/c0fb8d0b9c4d55770f7ce1a9cb62e4f2a75ca3ee/actionview/lib/action_view/helpers/translation_helper.rb#L65
(introduced in https://github.com/rails/rails/commit/0c7ac34aed1845044cd1911e5a775366d7ca41c1)