This makes mapgen v7 the deafult mapgen

Hi Jordach,
I know that this may be a little random, but I've added code that makes it so that mapgen 7 is the only mapgen version that can be used, and then removed the instructions that say to set mapgen v7 at world creation in the readme. It seems to all work, and I've not have problems after trying it with all the mapgen version currently available. 
Amaz.

Is that how a pull request should look like, or should it be more formal? I've never sent one before...