Install/configure PKGNG

PR includes:

* Cleanup/modernize unit test coverage ChefSpec 4.1+
* Cleanup/modernize integration test coverage for ServerSpec 2+
* Fix Rubocop and Foodcritic style errors
* Add a Rakefile for easy invocation of linting and specs
* Add a recipe for installing and configuring `pkg` (aka PKGNG) on pre-10 FreeBSD nodes.

/cc @opscode-cookbooks/release-engineers @scotthain 
@yzl So on FreeBSD 10+ `pkg` is installed and available. On pre-10 versions you need to install `pkg` and convert your package database to the new format.