Disable implicit mulitplexing in PHPMailerLite

Summary:
PHPMailerLite implicitly multiplexes mail, but we multiplex mail in an application-aware way higher in the stack. Disable the multiplexing.

The actual option is here: https://secure.phabricator.com/diffusion/P/browse/master/externals/phpmailer/class.phpmailer-lite.php;03dafec74f97bcd7$166

Test Plan: @klimek confirms this fixes his issue.

Reviewers: klimek, vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3365