semicolon react warning

Occasionally I get a React Warning about a semicolon. 

```
Warning: Style property values shouldn't contain a semicolon.
Check the render method of `ReactStickyState`. Try "marginTop: 0px" instead.
```

Seems to be related to [the use of fixedOffset](https://github.com/soenkekluth/react-sticky-state/blob/master/src/react-sticky-state.js#L289).