Do not use Enumerable in Registry

Calling methods like #first in Registry is misleading because it returns
a different result than registry.get(registry.keys.first).

lib/vagrant/registry.rb

@@ -4,8 +4,6 @@ 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

@@ -3,10 +3,6 @@ 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