Fixing PPLNS payouts when exceeding target

Round shares are taken to only match PPLNS target. Round target was not
re-adjusted to reflect the new, lower amount of round shares.

* Fix: Properly adjust round target shares when exceeding PPLNS target

Fixes #588 once merged
Merge pull request #595 from TheSerapher/issue-588

Fixing PPLNS payouts when exceeding target