fix(drops): Correct rarity tiers

Quick and dirty fix to Golden potions being more common than Zombie etc. Golden potions are now 2% of drops (down from 3%); Zombie and Cotton Candy potions each 2.33% of drops (up from 2%); Red, Shade, and Skeleton 3% of drops (unchanged); Base, White, and Desert 4% of drops (unchanged).

Fixes #169.