Use /bin/kill to stop sidekiq process instead of sidekickctl.

The example monitrc in the Sidekiq repository stops the process using kill, not sidekiqctl. I was having issues on my own EngineYard servers with monit spawning new Sidekiq processes a short time after killing them after a deploy with standard monit calls in deploy hooks. This lead to multiple Sidekiq instances running at once. This change fixed that in my recipes.