twig - security update

James Kettle, Alain Tiemblo, Christophe Coevoet and Fabien Potencier
discovered that twig, a templating engine for PHP, did not correctly
process its input. End users allowed to submit twig templates could
use specially crafted code to trigger remote code execution, even in
sandboxed templates.

For the stable distribution (jessie), this problem has been fixed in
version 1.16.2-1+deb8u1.

For the testing (stretch) and unstable (sid) distributions, this
problem has been fixed in version 1.20.0-1.

We recommend that you upgrade your twig packages.