fixes ChefRunner#default_cookbook_path

The method would fails to produce the correct path as soon as you start nesting spec files in subdirs under spec/.
I tried to stay as platform-agnostic as I could for this implementation (windows folks).

Lastly, I preserved this section of the original method, but I truly don't understand the point of it:

`caller(2).first.split(':').slice(0..-3).join(':')`

Wouldn't the following suffice?

`caller(2).first.split(':').first`

Perhaps there's some aspect of the `caller` method that I'm unaware of currently.