Use Reqwest backend for Appveyor, not Hyper which is deprecated

appveyor.yml

 environment:
   # When this was added there were revocation check failures when using the
   # libcurl backend as libcurl checks by default, but rustup doesn't provide the
-  # switch to turn this off. Switch to Hyper which looks to not check for
+  # switch to turn this off. Switch to Reqwest which looks to not check for
   # revocation by default like libcurl does.
-  RUSTUP_USE_HYPER: 1
+  RUSTUP_USE_REQWEST: 1
   CARGO_HTTP_CHECK_REVOKE: false
   matrix:
   - TARGET: x86_64-pc-windows-gnu