Upgrade Debian and Node versions

Summary:
Etherpad now requires nodejs >= 6.9.0 and recommand nodejs >= 8.9.

We can install Node from NodeSource Node.js binary distributions.

To use fresher dependencies, this change switches to Debain Stretch,
the current stable version.

Test Plan: Build image, run server for Nasqueron

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: http://devcentral.nasqueron.org/D1714