Raise CMake requirement to 3.0+

FindPythonLibs.cmake from CMake 2.8.x is not capable of finding Python
libraries on Maui, updating to CMake 3.x solves the problem.

Looks like it is better crafted.

Other parties interested in Calamares should already have CMake 3.x
so this is not a problem.

CMakeLists.txt

@@ -1,5 +1,5 @@
 project( calamares )
-cmake_minimum_required( VERSION 2.8.12 )
+cmake_minimum_required( VERSION 3.0 )
 set( CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/CMakeModules" )
 set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wl,--no-undefined" )
 set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wl,--fatal-warnings -Wnon-virtual-dtor -Woverloaded-virtual -Werror=return-type" )