CMake not installed by install-build-deps.sh on Ubuntu 16.04

@knarfS reports:

> When building AppImageKit the `install-build-deps.sh` only checks for Ubuntu 14.04 to install a newer version of `cmake`, but shouldn't check it also for 16.04, because 14.04 LTS has expired in April 2019. I guess more and more people will use 16.04 instead? And what's about Ubuntu 32bit (you have to compile a newer `cmake` yourself)?

To be honest, I never build locally, we have a quite involved setup using Docker and CentOS 6 for this repository to ensure increased binary compatibility with older systems. So yes, `install-build-deps.sh` should probably be updated.

> Shouldn't the build process work with the `cmake` version that is supplied by Ubuntu 14.04/16.04?

This is a question for @TheAssassin: Is the `cmake` version that is supplied by Ubuntu 16.04 sufficient to build AppImageKit?

> AppImageKit needs cmake version 3.6, 16.04 comes with 3.5.1

Can we make it so that 3.5.1 is sufficient? If not, why not?