change default python version on readme.md
change default python version on readme.md

According to CMakeLists.txt, default python version is 3.5.2, not 2.7.12
thewtex/python-cmake-buildsystem,python-cmake-buildsystem/python-cmake-buildsystem,thewtex/python-cmake-buildsystem,davidsansome/python-cmake-buildsystem,adrianbroher/python-cmake-buildsystem