cross-ci/lib/var/YEAR.js

`cross-ci` standardizes environment variables for CI. For example, your can simply use `BUILD_BRANCH` variable in all CI runners instead of `CIRCLE_BRANCH` in CircleCI or `TRAVIS_PULL_REQUEST_BRANCH` in Travis.

/// Current year as a four character long string.
const YEAR = () => (new Date()).toJSON().substr(0, 4);

module.exports = YEAR;