Fix compatibility with Sidekiq v6.5.7 (#366)

We don't use the process count parameter so we just set a default value
so that this works with previous versions too

Closes #365

lib/sidekiq/cron/poller.rb

@@ -43,7 +43,7 @@ def enqueue_job(job, time = Time.now.utc)
         handle_exception(ex) if respond_to?(:handle_exception)
       end
 
-      def poll_interval_average
+      def poll_interval_average(process_count = 1)
         @config[:cron_poll_interval] || POLL_INTERVAL
       end
     end