set remember token when sign out

app/helpers/sessions_helper.rb

@@ -21,7 +21,7 @@ module SessionsHelper
   end
 
   def sign_out
-    # current_user.update_attribute(:remember_token, User.digest(User.new_remember_token))
+    current_user.update_attribute(:remember_token, nil)
     cookies.delete(:remember_token)
     self.current_user = nil
   end