[CMake] update minimum version

Recent CMake versions added this warning:
CMake Deprecation Warning at CMakeLists.txt:6 (CMAKE_MINIMUM_REQUIRED):
  Compatibility with CMake < 2.8.12 will be removed from a future version of
  CMake.

And we are not compatible with CMake < 3.1:
ref https://github.com/jrl-umi3218/jrl-cmakemodules/issues/338

CMakeLists.txt

@@ -3,7 +3,7 @@
 # Copyright (c) 2018-2020 INRIA
 #
 
-CMAKE_MINIMUM_REQUIRED(VERSION 2.6)
+CMAKE_MINIMUM_REQUIRED(VERSION 3.1)
 
 SET(PROJECT_NAME eigenpy)
 SET(PROJECT_DESCRIPTION "Bindings between Numpy and Eigen using Boost.Python")