Auto merge of #247 - servo:jdm-patch-18, r=asajeffrey

Use VS 2019 in appveyor.

This matches the rest of our CI now.

appveyor.yml

@@ -1,5 +1,5 @@
 version: 1.0.{build}
-image: Visual Studio 2017
+image: Visual Studio 2019
 
 matrix:
   fast_finish: true
@@ -49,7 +49,6 @@ environment:
 install:
   - ps: Start-FileDownload "http://servo-rust.s3.amazonaws.com/build/MozillaBuildSetup-2.2.0.exe"
   - ps: .\MozillaBuildSetup-2.2.0.exe /S | Out-Null
-  - appveyor-retry choco install llvm --version=8.0.1
   - 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