Add :on_no_matching_servers to capistrano tasks.

If Capistrano does not find a matching server for a delayed_job task it stops execution, but sometimes you change your deploy script and there are no matching servers for a delayed_job task but you still want to keep the task there. 

It is possible to tell Capistrano to continue adding the option `:on_no_matching_servers => :continue` and the task will be just skipped `** skipping 'delayed_job:restart' because no servers matched`.