MySQL port should be always integer (HHVM compatibility)

This is only the change (so far), that is needed for compatibility with HHVM. 

PR solves this error message from HHVM:
```
Fatal error: Argument 5 to mysqli_connect() must be of type ?int, string given in /var/www/piwik/core/Tracker/Db/Mysqli.php on line 75
```