Replace Nanoc's LinkTo with Rails' UrlHelper

Nanoc's link helpers expect classes to be provided in a string but now
we're using arrays internally throughout so it makes sense to use the
Rails version.

guide/lib/helpers.rb

@@ -16,8 +16,8 @@ class Application < Rails::Application; end
 
 Dir.glob(File.join('./lib', '**', '*.rb')).sort.each { |f| require f }
 
+use_helper ActionView::Helpers::UrlHelper
 use_helper Nanoc::Helpers::Rendering
-use_helper Nanoc::Helpers::LinkTo
 use_helper Nanoc::Helpers::XMLSitemap
 use_helper Helpers::LinkHelpers
 use_helper Helpers::TitleAnchorHelpers