Make Registry enumerable

Registry already responds to #each, so including the Enumerable module
gives us nice methods like #select and #collect fo' free!

lib/vagrant/registry.rb

@@ -4,6 +4,8 @@ module Vagrant
   # This allows certain components (such as guest systems, configuration
   # pieces, etc.) to be registered and queried, lazily.
   class Registry
+    include Enumerable
+
     def initialize
       @items = {}
       @results_cache = {}

test/unit/vagrant/registry_test.rb

@@ -3,6 +3,10 @@ require File.expand_path("../../base", __FILE__)
 describe Vagrant::Registry do
   let(:instance) { described_class.new }
 
+  it "should include enumerable" do
+    expect(instance).to be_a(Enumerable)
+  end
+
   it "should return nil for nonexistent items" do
     expect(instance.get("foo")).to be_nil
   end