Simplify yaml format

[sheax0r]

Currently we are using this (note the extra indentation on "checklists"):

List:
  cards:
  - Card:
      checklists:
      - Checklist
        - 1
        - 2

Switch to this:

List:
  cards:
  - Card:
    checklists:
    - Checklist
      - 1
      - 2

This actually results in slightly different yaml - the original version produces {'Card'=>{'checklists'=>[1,2]}}, but the new version produces {'Card'=>nil,'checklists'=>[1,2]}, which is a bit weird but still results in a more readable file.