cmake_minimum_required(VERSION 3.7.2) & C++14

[moriarty]

• Bump the minimum CMake version, to fix a CMake warning.
• Remove explicit CMAKE_CXX_FLAGS c++0x because the default is now C++14

I recently enabled melodic tests on build.ros.org and #63 had a CMake warning. http://build.ros.org/view/Idev/job/Mdev__fetch_gazebo__ubuntu_bionic_amd64/