Add i18n-tasks for missing/unused translations

* i18n-tasks looks for missing *or* unused translations
* Add config file to ignore certain i18n keys (eg: activerecrord) which
  are used as defaults but not explicitly in views or helpers
* Remove code to raise in test env when there are unused translations
We are using it as a spec in my current project and it has worked quite well since it fails Travis when there are missing or unused translations. Moving to the default rake task would work, but I haven't used `i18n-tasks` that way so I wonder if it would make sense to try it on a project before adding it to Suspenders that way...