Use Python 2.7.13 in AppVeyor
Use Python 2.7.13 in AppVeyor

So we're testing the latest version.
terrelln/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard