Pass in props as an object

We're passing in an array here as "props"
https://github.com/shakacode/react-webpack-rails-tutorial/blob/master/client%2Fapp%2Fbundles%2Fcomments%2Fstore%2FcommentsStore.js#L8

But props should be 

``` js
{
comments: '<the comments array>'
}
```

This requires updating the jBuilder.

@jbhatab knows what to do on this one.
