Update ore generation to Minecraft 1.8

Minecraft 1.8 uses the same procedure as 1.7, with a few small
differences:

* The main loop now stops at i < maxSize instead of i <= maxSize. This
seems to change nothing in the world. This can be explained by the
conditions further down, which apparantly always fail when i == maxSize.
* i / maxSize isn't calculated five times anymore, it is now calculated
once and saved to a local variable.

Mojang claims these changes significantly speed up ore generation:
https://twitter.com/SeargeDP/status/488715162086813696 . I guess that
this performance improvement comes from the <= to < change, which helps
with larger ores, like those new stone variants.
Update cave generation to Minecraft 1.8

Caves cut through the surface blocks now. Sand no longer appears on the
ceiling of caves, it is replaced by sandstone. Red sand still appears
for now, red sandstone doesn't exist yet in 1.7.
Update ravine generation to Minecraft 1.8

* DefaultMaterial.STONE is no longer hardcoded, fixes #306.
* Ravines, unlike caves, haven't been updated to cut through most
surface blocks in vanilla. Sand, mycelium and clay will still block
ravines.