cmake: bump minimum required version from 2.6 to 3.0
cmake: bump minimum required version from 2.6 to 3.0

To silence this warning:

  CMake Deprecation Warning at CMakeLists.txt:1 (cmake_minimum_required):
    Compatibility with CMake < 2.8.12 will be removed from a future version of
    CMake.

bjornfor/fgui