Update to Node 10, update to Jekyll 3.6.3

[katydecorah]

This PR updates the Node version for this repository to 10.x. It also fixes security issues by updating Jekyll and other dependencies.

Ref https://github.com/mapbox/documentation/issues/377

• [x] update .artifacts.yml to use node_version: 10.x
• [x] update or add to package.json to use "engines": {"node": ">=10"}
• [x] update .travis.yml to use 10, example * [ ] update circle to use 10, example
• [x] update or add .nvmrc with 10.18.1 as the value of the file
• [x] after previous step, run nvm use && npm rebuild (or yarn install --force) and commit any changes made to the package/yarn-lock * [ ] if the application has CloudFormation templates: update Lambda runtimes to nodejs10.x
• [x] force push to publisher-staging
• [x] update readme to include requirements

[katydecorah]

@danswick since we make few updates to this repo, I included security updates for Jekyll and a few other devdependencies.