Action required: Greenkeeper could not be activated 🚨

🚨 You need to enable Continuous Integration on Greenkeeper branches of this repository. 🚨

To enable Greenkeeper, you need to make sure that a [commit status](https://help.github.com/articles/about-statuses/) is reported on all branches. This is required by Greenkeeper because it uses your CI build statuses to figure out when to notify you about breaking changes.

Since we didn’t receive a CI status on the [`greenkeeper/initial`](https://github.com/basarat/demo-sort-string/commits/greenkeeper/initial) branch, it’s possible that you don’t have CI set up yet.
We recommend using:
- [CircleCI](https://circleci.com)
- [Travis CI](https://travis-ci.org)
- [Buildkite](https://buildkite.com/)
- [CodeShip](https://codeship.com)
- [Azure Pipelines](https://azure.microsoft.com/en-us/services/devops/pipelines)
- [TeamCity](https://www.jetbrains.com/teamcity)
- [Buddy](https://buddy.works)
- [AppVeyor](https://www.appveyor.com)
But Greenkeeper will work with every other CI service as well.

If you _have_ already set up a CI for this repository, you might need to check how it’s configured. Make sure it is set to run on all new branches. If you don’t want it to run on absolutely every branch, you can whitelist branches starting with `greenkeeper/`.

Once you have installed and configured CI on this repository correctly, you’ll need to re-trigger Greenkeeper’s initial pull request. To do this, please click the 'fix repo' button on [account.greenkeeper.io](https://account.greenkeeper.io).

Start by checking whether the greenkeeper/initial branch receives a commit status from the repository's CI, then review the CI service configuration for all branches or greenkeeper/* branches. After configuring CI, use Greenkeeper's account “fix repo” action to re-trigger the initial pull request and confirm the branch status is reported.

Written by the indexing model from the issue text.