GitHub API errors

### Before submitting an issue, please be sure to

- [x] Read [the contributing instructions](https://github.com/github/pages-gem/blob/master/CONTRIBUTING.md#contributing-to-the-github-pages-gem)
- [x] Update to the latest Gem version (either `gem update github-pages` or `bundle update github-pages`)

### This issue affects

- [ ] The site generated by GitHub Pages
- [x] Building sites locally

### What did you do (e.g., steps to reproduce)

1.) `bundle exec jekyll s`
2.) make a file change

### What happened?

```
GitHub Metadata: No GitHub API authentication could be found. Some fields may be missing or have incorrect data.
GitHub Metadata: GET https://api.github.com/repos/akudrna/akudrna.github.io/pages: 404 - Not Found // See: https://developer.github.com/v3
GitHub Metadata: GET https://api.github.com/repos/akudrna/akudrna.github.io/pages: 404 - Not Found // See: https://developer.github.com/v3
```

I don't believe I'm using anything that requires authentication.

I know that this is technically a [jekyll/github-metadata](/jekyll/github-metadata) issue, but info about this issue should be in this readme as well, because the `jekyll_plugins` group includes it by default and users might have a hard time figuring out where the message comes from.

I'm not an owner of that repository, but I'm a collaborator and have write access. I tried generating a token selecting the whole `repo` scope and setting it as `JEKYLL_GITHUB_TOKEN`, which only removed the first line of the error.

I found [this solution](https://talk.jekyllrb.com/t/github-metadata-no-github-api-authentication-could-be-found-some-fields-may-be-missing-or-have-incorrect-data/2602/4) on Jekyll Talk, I haven't tried it yet and decided to try raising this issue first.

I was trying to find out what the `jekyll_plugins` Bundler group does, all I could find was https://github.com/github/pages-gem/pull/255#issuecomment-198465076, but I would like to know what **exactly** it does.