add libffi dep for debian to chefdk

Building chefdk on debian fails the [health check](https://gist.github.com/jordane/a3b75da42d0a0c4ca410) with a dependency on libffi. I'm not entirely sure why -- I tried playing around with some of the gem commands in the builds, but the only thing that worked for me was adding a direct dep on libffi (even though chefdk depends on berkshelf which depends on libffi).