Bump appveyor rust version to 1.50.0.

Author: AltSysrq

appveyor.yml

@@ -4,7 +4,7 @@ environment:
   - TARGET: nightly-i686-pc-windows-msvc
   - TARGET: nightly-x86_64-pc-windows-gnu
   - TARGET: nightly-i686-pc-windows-gnu
-  - TARGET: 1.36.0-x86_64-pc-windows-gnu
+  - TARGET: 1.50.0-x86_64-pc-windows-gnu
 install:
   - ps: Start-FileDownload "https://static.rust-lang.org/dist/rust-${env:TARGET}.exe" -FileName "rust-install.exe"
   - ps: .\rust-install.exe /VERYSILENT /NORESTART /DIR="C:\rust" | Out-Null