Fix :svn download errors with :latest casks

Library/Homebrew/cask/cask.rb

@@ -129,6 +129,10 @@ module Cask
       metadata_main_container_path/"config.json"
     end
 
+    def checksumable?
+      url.using.blank? || url.using == :post
+    end
+
     def download_sha_path
       metadata_main_container_path/"LATEST_DOWNLOAD_SHA256"
     end
@@ -136,12 +140,15 @@ module Cask
     def new_download_sha
       require "cask/installer"
 
+      # Call checksumable? before hashing
       @new_download_sha ||= Installer.new(self, verify_download_integrity: false)
                                      .download(quiet: true)
                                      .instance_eval { |x| Digest::SHA256.file(x).hexdigest }
     end
 
     def outdated_download_sha?
+      return true unless checksumable?
+
       current_download_sha = download_sha_path.read if download_sha_path.exist?
       current_download_sha.blank? || current_download_sha != new_download_sha
     end

Library/Homebrew/cask/installer.rb

@@ -397,7 +397,7 @@ module Cask
     end
 
     def save_download_sha
-      @cask.download_sha_path.atomic_write(@cask.new_download_sha)
+      @cask.download_sha_path.atomic_write(@cask.new_download_sha) if @cask.checksumable?
     end
 
     def uninstall