…op in CarouselItems

See reactstrap#1536

Resolves issue where if the UncontrolledCarousel contains CarouselItems with the same src, the following react error gets emitted:

```
Warning: Encountered two children with the same key, `https://xxx.png`. Keys should be unique so that components maintain their identity across updates. Non-unique keys may cause children to be duplicated and/or omitted — the behavior is unsupported and could change in a future version.
```

…item key

… node