[B] Fixes the Duration-Value in /effect command, Fixes BUKKIT-3967

The Issue:
When Using the /effect command. The Time is outputted in Ticks not in seconds.

PR Breakdown:
during the /effect command a variabnle (duration_sec) will be declared and initalized with duration / 20

JIRA-Ticket:
BUKKIT-3967 - https://bukkit.atlassian.net/browse/BUKKIT-3967