Using 'archive.php' is deprecated; also added nice

Original archive cron job caused the following error message:
```
Using this 'archive.php' script is no longer recommended.
Please use '/path/to/php /usr/share/piwik/console core:archive --url=https://piwik.XXX.org/piwik' instead.
To get help use '/path/to/php /usr/share/piwik/console core:archive --help'
See also: http://piwik.org/docs/setup-auto-archiving/
```

This pull request fixes see issue.

It also adds `nice` for this background task.