Fix bug where e-mails were not being sent out via Sidekiq

By default, ActionMailer uses the "mailers" queue, but this entry
was not included in the list of queues for Sidekiq to use.

For more details:

* https://github.com/plataformatec/devise/wiki/How-To:-Send-devise-emails-in-background-(Resque,-Sidekiq-and-Delayed::Job)
* http://guides.rubyonrails.org/active_job_basics.html

config/sidekiq_queues.yml

@@ -27,6 +27,7 @@
   - [gitlab_shell, 2]
   - [email_receiver, 2]
   - [emails_on_push, 2]
+  - [mailers, 2]
   - [repository_fork, 1]
   - [repository_import, 1]
   - [project_service, 1]