Hotfix #1: Fix entity collision damage with force field.

It seems that the CalclaviaCore CustomDamageSource.electrocution has
disappeared as it cannot be found by searching "electrocution."

Quick fix is to switch to the ModularForceFieldSystem.damagefieldShock
source. It should still fulfill its purpose and can always be changed
back.
Merge pull request #500 from Jamezz/master

Hotfix for #488: Fix entity collision damage with force field.