Add XSL extension

Summary:
The documentation generator phpDox requires xsl, cf. T641.

We so need to deploy xsl either here, either in nasqueron/jenkins-slave-php
image. It's probably generic enough to install it as base level.

Test Plan:
(1) Build the image
(2) php -i | grep -i xsl
(3) Try phpdox

Reviewers: dereckson

Maniphest Tasks: T641

Differential Revision: http://devcentral.nasqueron.org/D142