Switch mail sending to active_job

Before we were using the delayed_job method of delaying
mail sending which isn't working anymore on rails 4.

config/application.rb

@@ -64,5 +64,6 @@ module Osem
     # Errors raised within `after_rollback`/`after_commit` propagate normally
     # like in other Active Record callbacks.
     config.active_record.raise_in_transactional_callbacks = true
+    config.active_job.queue_adapter = :delayed_job
   end
 end