Problem: theme generation fails

The following error occurs if I try to use this theme:

```
warn: Filter "fileExists" is deprecated, use "fileExists(filename)"  
error: error while generating page "README.md": 

Template render error: (/tmp/book/_layouts/website/page.html)
  TypeError: Path must be a string. Received undefined
```

More details https://www.gitbook.com/book/sappo/rfc/build/574be31ce3ca0e1000bd6713
