When nodejs 8.9.0 was released, its bundled npm version was 5.5.1 (see
https://nodejs.org/en/download/releases). It makes sense that we lover our
requirement to that version.
Please note that the npm version mentioned here does not refer to the npm
library installed as Etherpad dependency in node_modules via package.json
(which indeed is higher) but is merely the npm version used to bootstrap the
installation when running installDeps.sh.
This change amends 9d35d15ae3 and its planning issue - #3424 - which were too
strict.
4f753809fe