tests: switch to Windows Server 2016 for AppVeyor builds. (#761)
tests: switch to Windows Server 2016 for AppVeyor builds. (#761)

Should hopefully fix the underlying bug that was causing tokio-tls tests to occasionally fail on Windows.

Signed-off-by: Toby Lawrence <c8420cae1b43ad0b08838661745c355024c86479@nuclearfurnace.com>
tokio-rs/tokio,tokio-rs/tokio