VERSION_GREATER_EQUAL is not implemented until CMake 3.7

[Piroro-hs]

CMake VERSION_GREATER_EQUAL macro is not implemented until CMake 3.7, so this line cause a problem (default CMake version on Ubuntu 16.04 is 3.5.1).

Change minimum required CMake version to 3.7, or change the condition to "${GAZEBO_VERSION}" VERSION_GREATER "8" OR "${GAZEBO_VERSION}" VERSION_EQUAL "8" should fix the issue.

[acxz]

Ubuntu 16.04 has reached EOL, as such this issue is not a priority.