Summary:
Make default init unavailable, as it will not initialise `YGNode` which is required for the setters
and getters of YGLayout's properties to work properly. In setters and getters we call the corresponding `YGNode`'s setters and getters. Thus the following code will crash
```
let layout = YGLayout()
layout.flexDirection = .row
```
Reviewed By: emilsjolander
Differential Revision: D6597796
fbshipit-source-id: 653d275d7314bfdd7a1106884fa3fbc0a13e1be7
dbf6a12134