Set cpack deb minimum version to 3.4

We have directives in the cpack deb builder in the engine which only work on cmake 3.4 or higher so we should set this as a minimum version. We then won't get bad packages on platforms such as Debian 8.

Specifically the post-inst scripts won't get bundled in before cmake 3.4.

We also need to document the following for Debian 8 builds:

sudo apt-get -t jessie-backports install "cmake"