AppVeyor change: use only Py3.6
AppVeyor change: use only Py3.6

Builds for 3.4 and 3.5 have been failing for ages; the latest
version is 3.6, let's try how if it work.

WojciechMula/pyahocorasick,pombredanne/pyahocorasick,pombredanne/pyahocorasick,WojciechMula/pyahocorasick,WojciechMula/pyahocorasick,pombredanne/pyahocorasick,pombredanne/pyahocorasick,WojciechMula/pyahocorasick