Update PHPMailer to 5.2.7 from 5.2.4.

Includes two trivial modifications for WordPress:
 * Doesn't use the autoloader, so the check to enforce the autoloader from the constructor is removed.
 * Requires class-smtp.php for backwards compatibility with direct (non-wp_mail()) uses of PHPMailer, as the autoloader isn't used.

props bpetty.
fixes #25560.


git-svn-id: https://develop.svn.wordpress.org/trunk@27385 602fd350-edb4-49c9-b593-d223f7449a82