downgrade cmake version for Centos7

[NiklasPhabian]

Since ce3f07e , STARE requires CMAKE 3.18.
Out of the box, yum on CentOS 7 provides us only with CMAKE 3.15.1 so building becomes one step more annoying.
STARE seems to build and complete all test(s) with cmake_minimum_required(VERSION 3.15). But I have obviously no idea of the implications.