cross-ci/lib/var/PROJECT_SLUG.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 PROJECT_SLUG = ({PROJECT_OWNER, PROJECT_NAME}) =>
    `${PROJECT_OWNER}/${PROJECT_NAME}`;

module.exports = PROJECT_SLUG;