Update logrotate file permissions

[greyvugrin]

This fixes 2 things:

1. Nginx has a bug where initial access.log and error.log files are owner by root (master process user) until they have been written to
2. create 0640 www-data www-data will create new files but the rotated ones are owned by root since that ran the process, unless you chown

Another option was to add su www-data www-data in here, but then it had issues rotating files, bc of the above :/.

Chowning in postrotate is the move, checks the boxes and keeps ownership as expected.