Use HTTPSFuture unconditionally on Windows

Summary: HTTPFuture doesn't work on Windows, but HTTPSFuture handles HTTP just fine (it doesn't handle high-precision timeouts and concurrency, which is why we use HTTPFuture when possible).

Test Plan: Created THIS VERY DIFF from Windows.

Reviewers: btrahan, Koolvin

Reviewed By: btrahan

CC: aran, epriestley, nh

Maniphest Tasks: T124

Differential Revision: https://secure.phabricator.com/D1783