Don't use "/e" in PHPMailer Lite

Summary: PHP 5.4 raises an E_STRICT warning about "/e", and it's a generally awful idea, even though this case doesn't look exploitable. Silence the warning by using preg_replace_callback() instead of "/e".

Test Plan: Sent myself a message with a bunch of multibyte UTF8 characters in it, it came through cleanly.

Reviewers: btrahan, vrana, jungejason

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1056

Differential Revision: https://secure.phabricator.com/D2147