Fix redirection to home on invalid token

Currently, when an invalid token is used, the redirection is supposedly done to the home page, but the function base_path() is used.
This function returns the server path (e.g. /var/www/laravel or /home/userx/my-site/) and not the URL path.
This resulting redirection produces a 404.

Using a simple "/" is sufficient for Laravel to redirect to the base URL of the install.