cross-ci/lib/var/MONTH.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 month numeric value as a string of length two.
const MONTH = () => (new Date()).toJSON().substr(5, 2);

module.exports = MONTH;