Always update to latest Windows base OS images

StefanScherer · StefanScherer · commit 49cccf06bd98 · 2017-07-23T09:43:40.000+02:00
diff --git a/appveyor.yml b/appveyor.yml
@@ -2,4 +2,6 @@ version: 1.0.{build}
 image: Visual Studio 2017
 
 build_script:
+  - docker pull microsoft/windowsservercore
+  - docker pull microsoft/nanoserver
   - ps: .\test-build.ps1
