appveyor: disable multithreading
appveyor: disable multithreading

Probably it crashes because of multithreading:

https://ci.appveyor.com/project/starius/lua-npge/build/0.0.1.11-test/job/cac8mty3a2agtodh#L726

starius/lua-npge,starius/lua-npge,npge/lua-npge,npge/lua-npge,starius/lua-npge,npge/lua-npge