Adding cmake minimum required version to file, otherwise cmake fails

Can't compile without this, otherwise you get:

    CMake Warning (dev) in CMakeLists.txt:
      No cmake_minimum_required command is present.  A line of code such as

	cmake_minimum_required(VERSION 2.8)

      should be added at the top of the file.  The version specified may be lower
      if you wish to support older CMake versions for this project.  For more
      information run "cmake --help-policy CMP0000".
    This warning is for project developers.  Use -Wno-dev to suppress it.