[B] Fix incorrect effect duration. Fixes BUKKIT-3967

The Issue:

Using the /effect command displays an incorrect duration. The displayed duration is in ticks.

PR Breakdown:

I changed the output from 'duration' for the seconds to 'NumberConversions.floor(duration / 20D)' in order to produce the correct time in seconds to the user.

Testing Results:

As expected, this fixed the incorrect effect duration. (Pictures) http://imgur.com/SS5zlx7,jCcF8yL,9Nc7pMw#0

Bukkit Ticket:

BUKKIT-3967: https://bukkit.atlassian.net/browse/BUKKIT-3967