fix(World group type): remove the children field

According to the discussion with Santiago:
- Organizations are not children of World. Organisations have no parent

Chat log:
https://monimus.hipchat.com/history/room/269039/2013/11/30?q=Organizations+are+not+children#02:30:10
Merge pull request #68 from mykabam/improve-world-group-type

fix(World group type): remove the children field