Slightly bump required CMake version to 3.4
Slightly bump required CMake version to 3.4

In our CMake scaffold for the OpenSSL module, we make use of the
imported targets OpenSSL::SSL and OpenSSL::Crypto. They are only
available in CMake ≥ 3.4 (released Sep 2015).

actor-framework/actor-framework,DavadDi/actor-framework,actor-framework/actor-framework,actor-framework/actor-framework,DavadDi/actor-framework,DavadDi/actor-framework,DavadDi/actor-framework,actor-framework/actor-framework