Change layout from nil to null to prevent warning

Since upgrading my Octopress blog (or perhaps Jekyll), I get the following warnings when building:

```
     Build Warning: Layout 'nil' requested in blog/categories/git/atom.xml does not exist.
     Build Warning: Layout 'nil' requested in blog/categories/ruby/atom.xml does not exist.
     Build Warning: Layout 'nil' requested in blog/categories/rails/atom.xml does not exist.
     Build Warning: Layout 'nil' requested in blog/categories/http/atom.xml does not exist.
     Build Warning: Layout 'nil' requested in blog/categories/chrome/atom.xml does not exist.
```

This is fixed by changing all instances of `layout: nil` to `layout: null`. 

See https://github.com/jekyll/jekyll-help/issues/112 where this is discussed further.

Love the theme by the way, just updated my blog to use it! Thanks!