This change fixes the following issue:

ButtonGroup was adding positionInGroup to all children rather than just Buttons.  For example, in the following, both the Buttons and the child spans get positionInGroup props:

```
<Button.Group><Button><span>b1</span></Button><Button><span>b2</span></Button></Button.Group>
```

This results in a React warning: “Warning: React does not recognize the `positionInGroup` prop on a DOM element...”

The review comment in the original code change causing this hints at this issue as well:

themesberg#1273 (comment)