move dependencies into metadata.rb

When iterating a Cookbook the `metadata.rb` file should be the authoritative source for dependencies. It's always best to put all of your dependencies in there much like you would for your RubyGems in a gemspec file.

It's a good idea to use the `Berksfile` to overwrite the "location" of a Cookbook. It's also a good idea to put any development or test dependencies in the `Berksfile` which you wish your developers / testers to pull down but you do not want an end user to get. This is possible because end users will not resolve the Berksfile - they'll just resolve the metadata.rb of a dependency.

This is the same behavior of the Gemfile and Gemspec in Rubygems