emacs: 25.2 -> 25.3

https://lists.gnu.org/archive/html/emacs-devel/2017-09/msg00211.html

> This is an emergency release to fix a security vulnerability in Emacs.
>
> Enriched Text mode has its support for decoding 'x-display' disabled.
> This feature allows saving 'display' properties as part of text.
> Emacs 'display' properties support evaluation of arbitrary Lisp forms
> as part of instantiating the property, so decoding 'x-display' is
> vulnerable to executing arbitrary malicious Lisp code included in the
> text (e.g., sent as part of an email message).