Minor css typo in switch-handle

The css class rendered for the `switch-handle` span has a typo in it:

```html
class="switch-handle btn btn-lightbtn-sm"
```

This is due to a typo in src/bootstrap-switch-button-react.js near line 114 (in master f206c5bf5b2):
```jsx
<span className={'switch-handle btn btn-light' + (this.state.size ? 'btn-' + this.state.size : '')} />
```

I will submit a PR.