Prevent anon/banned/member users from voting (fix #2719).

There was a regression in 6d6d00b; `before_filter :voter_only` was a
no-op in the post vote controller because it merely returned false,
which does not halt the request. The fix is to arrange for a voter_only
method to be defined that properly redirects to the access denied page.

app/controllers/application_controller.rb

@@ -133,9 +133,9 @@ protected
     end
   end
 
-  %w(member banned builder gold platinum janitor moderator admin).each do |level|
-    define_method("#{level}_only") do
-      if !CurrentUser.user.is_banned_or_ip_banned? && CurrentUser.user.__send__("is_#{level}?")
+  User::Roles.each do |role|
+    define_method("#{role}_only") do
+      if !CurrentUser.user.is_banned_or_ip_banned? && CurrentUser.user.__send__("is_#{role}?")
         true
       else
         access_denied()