ENH: update cmake minimum required in tutorial

Documentation/SoftwareGuide/Latex/Tutorial.tex

@@ -39,7 +39,7 @@ Open the \code{CMakeLists.txt} file and write in the few lines:
 \begin{cmakecode}
 PROJECT(Tutorials)
 
-cmake_minimum_required(VERSION 2.6)
+cmake_minimum_required(VERSION 3.1.0)
 
 FIND_PACKAGE(OTB)
 IF(OTB_FOUND)
@@ -102,7 +102,7 @@ Follow the following steps:
 \begin{cmakecode}
 project(MyFirstProcessing)
 
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 3.1.0)
 
 find_package(OTB REQUIRED)
 include(${OTB_USE_FILE})