Update instructions, Dockerfile and Travis CI to use Node.js 10.
Specify that Node.js version 10 or greater is needed in the engines
section of package.json (note that this is only advisory.) And document
this as a breaking change in CHANGELOG.md.
This is a backport of 69a1ff7.
Add --only=production flag to Dockerfile and the README entries
describing how to install nodes. This will avoid pulling in a lot of
dependencies that are not needed for running master/slaves.
Hornwitser