speed up AppVeyor by skipping commits unrelated to build/test correctness and build for py36 only; test ti command on Windows
speed up AppVeyor by skipping commits unrelated to build/test correctness and build for py36 only; test ti command on Windows

yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi