Update version to 2.6.1-SNAPSHOT
New chunk cache

During chunk population, population is now restricted to only four
chunks, (x,z), (x+1,z), (x,z+1) and (x+1,z+1). This makes terrain
population much more consistent, as it won't depend anymore on the
direction terrain generated in. Bugs that were previously only exposed
when travelling in one direction, should now be exposed in all
directions.

Also replaced some chunkX, chunkZ pairs with ChunkCoordinate.
Add new sapling types, drop ancient sapling ids

- Added new sapling types for the saplings added in Minecraft 1.7.
- Removed sapling ids. The ids were deprecated as of
2ba14a8d9c6b6645e5dc2a426f8be6cad6c78c5d, and TC 2.4, 2.5 and 2.6.0 have
autoconverted the settings. If needed, I can add support back.
- Fixed a bug with 2x2 sapling formations. TC will now correctly place
the BO3.