Update for Berkshelf Workflow

I've made a few changes that make this cookbook compatible with a Berkshelf workflow.  Any thoughts?
  
Changes: 
* Use test-kitchen, kitchen-vagrant and berkshelf from rubygems. Require berkshelf 3.
* Use updated syntax for Berksfile.
* Replace recommends with depends in metadata.
* proxy.rb recipe determines which proxy to use via a node attribute, and includes either nginx or openresty automatically.  This fixes an issue with the cookbook throwing errors if the proxy.rb recipe is included in another recipe, rather than explicitly in the run list.