Placed the task in onEnable()

By having the scheduled task triggered by PlayerJoinEvent, you create a
new task every time someone joins the server. These tasks will keep
running until your server eventually has so many tasks it runs itself to
death. :) Also, 20L is once per second. That is probably a tad overkill. May once every 100L or 200L (5-10 seconds should do.) :P