Revert back to C++11 for CentOS 7
CentOS 7 uses g++ 4.8.5, which doesn't do c++14. Had to revert use of make_unique to support CentOS runners. Considered cutting CentOS 7 support, but it's the only RHEL/Cent OS we support running on. Maybe can deprecate CentOS 7 support when we get CentOS 8 or RHEL up.