expand S3 shorthand urls in authenticate_box_url hook

Vagrant 1.8 only supports HTTP metadata boxes. Expand s3:// shorthand
URLs in the authenticate_box_url hook so that Vagrant's metadata check
sees HTTP URLs. This method should also be compatibile with Vagrant
1.5+.

Fix #27.

.travis.yml

@@ -1,6 +1,6 @@
 language: ruby
 rvm:
-- 2.1
+- 2.2.3
 
 before_install:
 # Install Bats, the Bash testing framework
@@ -34,7 +34,8 @@ env:
   - VAGRANT_S3AUTH_BOX_BASE=minimal
   matrix:
   - VAGRANT_VERSION=master
-  - VAGRANT_VERSION=v1.7.1
+  - VAGRANT_VERSION=v1.8.1
+  - VAGRANT_VERSION=v1.7.4
   - VAGRANT_VERSION=v1.6.5
   - VAGRANT_VERSION=v1.5.1
 

lib/vagrant-s3auth/middleware/expand_s3_urls.rb

@@ -0,0 +1,28 @@
+require 'uri'
+
+module VagrantPlugins
+  module S3Auth
+    class ExpandS3Urls
+      def initialize(app, _)
+        @app = app
+      end
+
+      def call(env)
+        env[:box_urls].map! do |url_string|
+          url = URI(url_string)
+
+          if url.scheme == 's3'
+            bucket = url.host
+            key = url.path[1..-1]
+            raise Errors::MalformedShorthandURLError, url: url unless bucket && key
+            next "http://s3.amazonaws.com/#{bucket}/#{key}"
+          end
+
+          url_string
+        end
+
+        @app.call(env)
+      end
+    end
+  end
+end

lib/vagrant-s3auth/plugin.rb

@@ -17,6 +17,11 @@ class Plugin < Vagrant.plugin('2')
       description <<-DESC
         Use versioned Vagrant boxes with S3 authentication.
       DESC
+
+      action_hook(:s3_urls, :authenticate_box_url) do |hook|
+        require_relative 'middleware/expand_s3_urls'
+        hook.prepend(ExpandS3Urls)
+      end
     end
   end
 end