Run delayed job tasks in the staging environment

We added the background role to the production and preproduction
environments in commit d0b0782c4, but forgot to add it to the staging
environment as well.

config/deploy/staging.rb

@@ -1,3 +1,3 @@
 set :branch, ENV["branch"] || :master
 
-server deploysecret(:server), user: deploysecret(:user), roles: %w[web app db importer cron]
+server deploysecret(:server), user: deploysecret(:user), roles: %w[web app db importer cron background]