Re-enabled ChunkGC by default.

ChunkGC was previously disabled due to a bug with IC2. However, I realized
that ChunkGC was not the issue and that the IC2 bug was actually caused by
keepSpawnInMemory which I fixed in this commit :

MinecraftPortCentral/Cauldron@19253e084ac8f687adc061bb3d7943c33b29a6eb