Update Extreme Hills

The new mountains have made extreme hills obsolete. They're one of the ugliest biomes and I feel disappointed when I see it in the distance. With the world height being raised, just like clouds being so low, the coldness of the biome (indicated by the grass hue) doesn't really make much sense anymore. They should appear more "hill like" with a new resource called slate (google "slate wales" for some cool slate hills).