* Travis CI: add Node.js 10.x.
* Move eslint mocha where it's needed.
* Update devDependencies.
Notes:
* package-lock.json is saved with npm 6.3.1, the one that ships with node.js 10.x
* eslint 5.x doesn't support node.js 4.x, so don't update it
* Refactor package.json properties.
* Update Travis config.
* test node.js node 8 and remove 5 and 4.0
* cache the node_modules folder
* switch to double quotes
* Update dependencies.
* Add package-lock.json.
* Clean up .gitignore to the bare minimum.