cross-ci/lib/var/IS_PR.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 envCi = require('../envCi');

/// Boolean, `true` if the current build is triggered by a pull request.
const IS_PR = ({BUILD_PR_NUM}) =>
    Boolean(BUILD_PR_NUM)
    || envCi.isPr;

module.exports = IS_PR;