build: Update minimum required CMake version to 3.3.0

This is already the practical requirement (set by ABC).
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0b54a72..18a87b4 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 2.8.12)
+cmake_minimum_required(VERSION 3.3.0)
 
 project("VTR")