Fix high cpu usage while waiting next fetch time

[kenhys]

Closes: #221

Reported by tma-prismo

Before:

  until @finished
    if Time.now > @next_fetch_time
      ...
      sleep 1
    end
  end

After:

  until @finished
    if Time.now > @next_fetch_time
      ...
    end
    sleep 1
  end