also build with MSVC 2019

[skip ci]

appveyor.yml

@@ -7,11 +7,13 @@ branches:
   - /^appveyor/
   - /^build-ci/
 image:
+- Visual Studio 2019
 - Visual Studio 2017
 - Visual Studio 2015
 build_script:
 - cmd: >-
-    if "Visual Studio 2017"=="%APPVEYOR_BUILD_WORKER_IMAGE%" call "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvars64.bat"
+    if "Visual Studio 2019"=="%APPVEYOR_BUILD_WORKER_IMAGE%" call "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvars64.bat"
+      if "Visual Studio 2017"=="%APPVEYOR_BUILD_WORKER_IMAGE%" call "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvars64.bat"
       if "Visual Studio 2015"=="%APPVEYOR_BUILD_WORKER_IMAGE%" call "C:\Program Files\Microsoft SDKs\Windows\v7.1\Bin\SetEnv.cmd" /x64
       if "Visual Studio 2015"=="%APPVEYOR_BUILD_WORKER_IMAGE%" call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" x86_amd64
       cd..