Remove curl usage

Author: Diggsey

appveyor.yml

@@ -22,7 +22,7 @@ branches:
 
 install:
   # Install rust, x86_64-pc-windows-msvc host
-  - curl -sSf -o rustup-init.exe https://win.rustup.rs/
+  - appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
   - rustup-init.exe -y --default-toolchain=nightly-2017-01-06-x86_64-pc-windows-msvc
   - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin