Rails Devise - how to open access to the site root - ruby-on-rails

Question

I have a Rails app that acts as a backend for an iOS app. It was developed by a third party, and I've never used Rails, so trying to learn as I go here. I want to make the app web-accessible for users, with a homepage front-end for giving prospective users some info about the app.
Sounds easy enough, but the app is using Devise for user authentication and when I hit the site root I get the following error message:
You need to sign in or sign up before continuing.
In the routes.rb file I have the following:
root :to => "visitors#index"
devise_for :users
resources :users
In my visitors controller I added a before_filter to try to open up the site index:
class VisitorsController < ApplicationController
before_filter :authenticate_user!, :except => [:index]
end
but this made no difference. Is there something obvious I'm missing here, or some concept that I'm not grasping? Basically I want a user to hit the site homepage, then click on a 'Login' link, after which they'll have access to the rest of the site.
EDIT:
My ApplicationController:
class ApplicationController < ActionController::Base
respond_to :html, :json
# Prevent CSRF attacks by raising an exception.
# For APIs, you may want to use :null_session instead.
protect_from_forgery with: :null_session, :if => Proc.new { |c| c.request.format == 'application/json'}
before_action :configure_permitted_parameters, if: :devise_controller?
acts_as_token_authentication_handler_for User
protected
def configure_permitted_parameters
devise_parameter_sanitizer.for(:sign_up) << :name
devise_parameter_sanitizer.for(:account_update) << :name
end
end

Answer 1

From Devise's how to guide which seems to describe your example. You can create two roots in your routes.rb file. One for authenticated user's and one for non-authenticated users:
authenticated :user do
root :to => 'visitors#some_action', :as => :authenticated_root
end
root :to => 'visitors#index'
However, please post your ApplicationController because your example code should work, the issue may be there.

Answer 2

In case anyone else has the same problem - the culprit was the following line in application_controller.rb:
acts_as_token_authentication_handler_for User
It's part of the simple_token_authentication Gem (https://github.com/gonzalo-bulnes/simple_token_authentication)
To fix enable anonymous access to the root page, I changed it to the following:
acts_as_token_authentication_handler_for User, :except => [:index]