Update Bukkit version to 1.7.8

Also updated the Forge version to the latest build, but Forge is still
on 1.7.2.
Fix ReplacedBlocks in virtual biomes

This slows down ReplacedBlocks a little bit, but at least TC maintains a
biome cache.
Add PopulationBoundsCheck to disable bounds checking during chunk generation

It still won't try to load new chunks, that could go horribly wrong.

This method relies on the assumption that cache misses will be rare, and
when they happen, WorldServer has another cache for the last accessed
chunk.