[5.2] Register swiftmailer before mailer

[jenssegers]

This change allows users to extend the swift.transport manager from their own service providers:

    public function register()
    {
        $this->app['swift.transport']->extend('postmark', function () {
            return new PostmarkTransport($this->app['config']->get('services.postmark.token'));
        });
    }

Without this it is not possible to add mail drivers before the mailer is constructed with the current mail driver, because swift.transport is not registered on the container yet.

This applies to all 5.x versions.