Update lib/tasks/sidekiq.rake

Mac OS uses launchd instead of /etc/init.d to start daemons and tasks to be started by launchd MUST NOT daemon itself. So "nohup" here won't work for Mac OS. 

Can we add a "launchd" task to the rake file so that we can start sidekiq as "bundle exec rake sidekiq:launchd" ?
Merge pull request #2969 from zerustech/patch-1

Update lib/tasks/sidekiq.rake