appveyor: switch to new VS/MinGW and x64 The cache had been reseted with the following REST API requests: DELETE https://ci.appveyor.com/api/projects/nmathewson/libevent/buildCache DELETE https://ci.appveyor.com/api/projects/libevent/libevent/buildCache DELETE https://ci.appveyor.com/api/projects/azat/libevent/buildCache