cross-ci/lib/var/JOB_NUM.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.

/// CI service job number
const envCi = require('../envCi');

const JOB_NUM = () => envCi.job;

module.exports = JOB_NUM;