application_controller.rb

class ApplicationController < ActionController::Base
  before_filter :ensure_proper_protocol

  protected

  def ssl_allowed_action?
    (params[:controller] == 'users/sessions' && ['new', 'create'].include?(params[:action])) ||
      (params[:controller] == 'users/registrations' && ['new', 'create', 'edit', 'update'].include?(params[:action])) ||
      (params[:controller] == 'users/omniauth_callbacks')
  end

  def ensure_proper_protocol
    if request.ssl? && !ssl_allowed_action?
      redirect_to "http://" + request.host + request.fullpath
    end
  end

  def after_sign_in_path_for(resource_or_scope)
    root_url(:protocol => 'http')
  end

  def after_sign_out_path_for(resource_or_scope)
    root_url(:protocol => 'http')
  end
end

registrations_controller.rb

class Users::RegistrationsController < Devise::RegistrationsController
  force_ssl :only => [:new, :create, :edit, :update]

  protected

  def after_inactive_sign_up_path_for(resource)
    root_url(:protocol => 'http')
  end

  def after_sign_up_path_for(resource)
    root_url(:protocol => 'http')
  end

  def after_update_path_for(resource)
    edit_user_registration_url(:protocol => 'http')
  end
end

sessions_controller.rb

class Users::SessionsController < Devise::SessionsController
  force_ssl :only => [:new, :create]
end