use old m2crypto to workaround test failure under python3
    
    The latest version (0.27) of m2crypto has a bug in setup.py which blocks
    the installation under python3 env, see:
    
    https://gitlab.com/m2crypto/m2crypto/commit/405d14ab7396ba570ecb68cc5cec52dc11424f27
    
    to workaround the test failure, just use the old version.