Inline chef version spec in Gemfile

Expect CHEF_VERSION to have the right form.
Vagrant: Allow comma as separator in RECIPES
Rename backports-sloppy recipe to backports_sloppy

Be consistent with other recipes.

Relates to #8
Do not include backports recipe on backports_sloppy on Squeeze

squeeze-backports-sloppy includes all squeeze-backports packages.
Include it still on Wheezy.
Use vendor/cookbooks for ChefSpec

`vendor/cookbooks` is automatically included to its cookbook_path
starting from v1.2.0.