Retry appveyor downloads

Author: alexcrichton

Diff for: appveyor.yml

@@ -24,7 +24,7 @@ environment:
 
 install:
   # Install rust, x86_64-pc-windows-msvc host
-  - appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
+  - appveyor-retry appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
   - rustup-init.exe -y --default-host x86_64-pc-windows-msvc
   - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin