Constant layout does not handle node updates

When node is updated with constant layout, it is removed. Check line 67 of src/Layout/constant.js.
I suggest to turn it into

``` javascript
if (change.changeType === 'add' || change.changeType === 'update')
```

Same goes for line 73.
When changed like that, I can successfully update my nodes.

Please comment and fix.

Start by inspecting src/Layout/constant.js at lines 67 and 73, then reproduce an update to a node using the constant layout. Verify that updated nodes are retained and that the behavior works at both locations.

Written by the indexing model from the issue text.