Use Sinatra's public_folder if it exists

Fixes resque#421. Fixes resque#420.

Author: defunkt

lib/resque/server.rb

@@ -13,7 +13,13 @@ class Server < Sinatra::Base
     dir = File.dirname(File.expand_path(__FILE__))
 
     set :views,  "#{dir}/server/views"
-    set :public, "#{dir}/server/public"
+
+    if respond_to? :public_folder
+      set :public_folder, "#{dir}/server/public"
+    else
+      set :public, "#{dir}/server/public"
+    end
+
     set :static, true
 
     helpers do