Update default .kitchen.yml file, leaner & cleaner.

In all of its glory, assuming you chose the kitchen-vagrant driver:

    ---
    driver: vagrant

    platforms:
      - name: ubuntu-12.04
      - name: centos-6.4

    suites:
      - name: default
        run_list:
          - recipe[mycookbook::default]
        attributes:
          kitchen: is_fun