appveyor: switch to Visual Studio 2017

ldorau · igchor · commit 7a2efe3c0413 · 2019-09-06T12:00:44.000+02:00
This change is required because of the following issue: https://help.appveyor.com/discussions/problems/24684-problem-with-vcpkg-wrong-version-on-appveyor appveyor/ci#3052 which cannot be fixed directly now.
diff --git a/appveyor.yml b/appveyor.yml
@@ -1,5 +1,5 @@
 version: 1.4.{build}
-os: Visual Studio 2015
+os: Visual Studio 2017
 platform: x64
 
 environment:
