fix download to track with upstream api change

lib/cask/download.rb

@@ -7,8 +7,12 @@ class Cask::Download
 
   def perform
     require 'software_spec'
-    software_spec = SoftwareSpec.new(cask.url.to_s, cask.version)
-    downloader = CurlDownloadStrategy.new(cask.title, software_spec)
+    cask = @cask
+    resource = Resource.new(cask.title) do |r|
+      r.url     cask.url.to_s
+      r.version cask.version
+    end
+    downloader = CurlDownloadStrategy.new(cask.title, resource)
     downloaded_path = downloader.fetch
 
     _check_sums(downloaded_path, cask.sums) unless cask.sums === 0