Healthcheck errors when there's no CNAME present

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

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

### This issue affects

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

*The GitHub Pages Gem is intended to help users replicate the GitHub Pages build environment locally. If your issue affects both the hosted version and the version previewed locally, you may be better suited reporting [seeking support in other forums](https://github.com/github/pages-gem/blob/master/docs/SUPPORT.md).*

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

Run `github-pages health-check` in my blog repo.

### What did you expect to happen?

No error about missing CNAME, since I've intentionally removed the file to switch from a custom domain to the github.io domain.

### What happened instead?

```
Error: No CNAME file found in current directory
```

### Additional information

The `pages-gem` will always raise if there's no CNAME. Shouldn't a CNAME be optional as not everyone will be using a custom domain? Perhaps it could be a warning instead.

https://github.com/github/pages-gem/blob/4c74bad4198735f59e48921ae01fe60749d4acc6/bin/github-pages#L46