Wyvern Healing Breath minor fix

[Casey]

The current LUA uses wyvern current hp instead of maximum HP to determine healing breath potency.

This change would make Wyvern HP+ gear useful again, as well as making healing breath more retail-like and not losing healing potency from AoE damage.

This will allow the closure of the bug I reported on the tracker:

http://bugs.dspt.info/show_bug.cgi?id=1056