Remove unused 'cmake_minimum_required' commands

Top level CMakeLists.txt already check '3.1.3' so there is no need to check
it for subdirectories

examples/CMakeLists.txt

-cmake_minimum_required(VERSION 3.1.3)
-
 # The examples need a few additional dependencies (e.g. boost filesystem and OpenCV highgui):
 
 #check installed version in order to include the correct OpenCV libraries

python/CMakeLists.txt

-cmake_minimum_required(VERSION 3.1.3)
-
 # The python bindings may need a few additional dependencies (e.g. boost filesystem and OpenCV), which are
 # included below - at some point it should be checked if they are really needed.
 

utils/CMakeLists.txt

-cmake_minimum_required(VERSION 3.1.3)
-
 # The utils need a few additional dependencies (e.g. boost filesystem and OpenCV highgui):
 
 #check installed version in order to include the correct OpenCV libraries