Change slime chunk spawning restrictions to use block material instead of hardcoded values

[ammoore00]

As found in Slime code written by FC:

private boolean canSpawnOnBlockInSlimeChunk(int i, int j, int k) {
		// FCTEST: this needs to be based on material, not specific block type.  Release as is.
		int iBlockID = worldObj.getBlockId(i, j, k);
		
		return iBlockID == Block.dirt.blockID || iBlockID == Block.stone.blockID || iBlockID == Block.grass.blockID || iBlockID == Block.gravel.blockID ||
				iBlockID == Block.sand.blockID;
	}