Add .slick-active class on slides that are visible

When reaching the end of a finite slider with multiple slides, not all of them do get the `.slick-active` class.

Here's a little snapshot to describe the issue.

The slider is set up as following:
```
slidesToShow={4}
slidesToSroll={4}
infinite={false}
```

Initial - first 4 slides visible
```

```

After click on next arrow (actual behavoir) - last 4 slides visible
```

```

After click on next arrow (expeted behavoir) - last 4 slides visible
```

```

Is this an expected bahvoir? If not, I've a fix ready and I'm happy to file a pull request.

Best

Roland