Bump NodeJS LTS 4.x.x as the minimum supported NodeJS version

[netweb]

Via ​#WP35105 Specify minimum node and npm version in package.json

WordPress will also be bumping to the 4.2.x LTS branch of NodeJS (​ref)

This ticket adds NodeJS version 4.2.1 as the minimum supported NodeJS version per the initial 4.2.x LTS branch release announcement ​here, see also ​https://github.com/nodejs/LTS/blob/master/README.md

Various NPM packages already require greater than the default Travis CI installed version 0.10.32:

• node-sass - "node": ">=0.10.40"
• grunt-check-dependencies - "node": ">=0.12.7 <0.13 || >=4.1.0"
• postcss - "node": ">=0.12
• stylelint - "node": ">=0.12
• autoprefixer - "node": ">=0.12

[netweb]

Example Travis CI job using 7008.diff​: ​https://travis-ci.org/ntwb/BuddyPress/jobs/122007969#L188

[djpaul]

In 10703:

Specify minimum node and npm version in package.json.

Some of the NPM packages we use have minimum versions of node. This can lead to notices when a developer does an npm install on a machine with an older version of NPM, or when the grunt-check-dependencies task runs.

Fixes #7008

Props netweb