www/drush: Update to 8.1.16

Changelog: https://github.com/drush-ops/drush/compare/8.1.15...8.1.16

*While here switch to DISTVERSION

PR: 225817
Submitted by: Matthias Fechner <idefix@fechner.net> (maintainer)
Approved by: mentors (implicit)
Differenial Revision: