Bugfix for PRE-Tag and Sub-Tags

Version 5.7.2 introduced a bug while trying to fix ”< (less-than sign)
inside a PRE element, and NOT start of a valid tag, was incorrectly
removed” - closing tags inside the pre-tag are now invalid and look
like e.g. “&lt;/span>”

This should be pushed upstream. (see http://www.mpdf1.com/forum/discussion/1823/new-bug-in-pre-handling-due-to-fix-in-5-7-2)