Specify a node/npm version

In order to easily move the project between environments (local dev, heroku, CI, etc.), I want to know which version of node and npm the project is meant to run under.

I know that heroku [looks for](https://devcenter.heroku.com/articles/nodejs-support#specifying-a-node-js-version) an `engines` section in the package.json, but I am unsure if this is the most portable solution.

Start with package.json and the issue's Heroku reference, then inspect the repository's existing environment and deployment configuration. Determine where the intended Node and npm versions should be specified for local development, Heroku, and CI; the work is done when those environments have a clear, portable version declaration.

Written by the indexing model from the issue text.