Perform a headless apt-get upgrade

I've read a few places that it isn't recommended to do an `apt-get upgrade` in a chef recipe, but this seems pretty innocuous to me. This isn't a dist-upgrade, so held packages like the kernel won't get upgraded. Also, the dpkg options mean that only non-modified conf files will be overwritten during upgrades. For any modified ones, the new config file will be written with the `.dpkg-dist` suffix. But, if we don't want to do this out of principal, we can just kill this PR.