Kirby error

Adding this plugin leads to error on Kirby.

## Settings
- Kirby v2.5.12
- basic StarterKit them for Kirby
- added tracking code as on readme on snippet `footer.php`:
```
<?= piwik(); ?>
</body>
</html>
```

## Error
```
session_start(): Cannot send session cache limiter - headers already sent
```

 
