Re-fixed Sim-environment-current-volume-malfunction-scaling bug.

The May 21, 2008 fix of a similar bug in StoreImpl.java suggests that
the intended scaling for severe, medium, and low permanent malfunctions
is volume reduction by 100%, 75%, & 50%, not by 100%, 50%, & 25% as
implemented in previous commit.

* Corrected current volume scaling for medium and low permanent
  malfunctions.