Add cmake_minimum_required to root CMakeLists.txt

There was already a minimum version specified in the actual CMakeLists
file in the C++ directory, but since the parent file does not have it
you get a warning if you build in the idiomatic cmake fashion:

    mkdir build
    cd build
    cmake .. # <---- warning here because no minimum version specified

I used 3.1 as min version because that is the same as specified in child
file.

CMakeLists.txt

+cmake_minimum_required(VERSION 3.1)
 add_subdirectory(c++)