cross-ci/lib/var/BRANCH_STAGING.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.

const BRANCH_STAGING = () => process.env.BRANCH_STAGING || 'master';

module.exports = BRANCH_STAGING;