suppress build warnings of jekyll 2.3

jekyll 2.3 changed yaml semantics, which introduces build warning for older configurations

```
  $ sudo docker logs james_blog

  Configuration file: /data/_config.yml
              Source: /data
         Destination: /var/www/html
        Generating...
       Build Warning: Layout 'nil' requested in atom.xml does not exist.
       Build Warning: Layout 'nil' requested in rss.xml does not exist.
```
see jekyll/jekyll#2712 for details