cross-ci/lib/var/IS_RELEASE.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.

/// Is `true` if currently built branch is one of `RELEASE_BRANCHES`.
const IS_RELEASE = ({RELEASE_BRANCHES, BUILD_BRANCH}) =>
    RELEASE_BRANCHES.indexOf(BUILD_BRANCH) > -1;

module.exports = IS_RELEASE;