README: Add apt-get update and upgrade to example

The current image includes nginx 1.4.3 which [has at least one security vulnerability](http://nginx.org/en/security_advisories.html).

These commands are necessary to ensure the latest package versions with security fixes are installed.

This change should probably also be applied to the baseimage README.

Something related: I'm not exactly sure how the Ubuntu apt sources work, but when looking at the `apt-get update` output, I missed the `security.ubuntu.com` sources which I usually see when updating an Ubuntu system.