drop CMake older than 3.5

It generates a warning with newer versions of CMake.
3.4 was released in 2015. We require C++17 with compilers only available in 2017/2018.
If your CMake is older than your toolchain, you have a problem.

There's no need to support legacy CMake going forward in libebml 2/libmatroska 2.

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.1.2)
+cmake_minimum_required(VERSION 3.5)
 
 project(matroska VERSION 2.0.0)