Change appveyor to use VS2017 image

This hopefully will prevent the increasing number
of SSL/TLS errors that we are seing during updates.

.appveyor.yml

@@ -4,7 +4,7 @@ version: '{build}'
 pull_requests:
   do_not_increment_build_number: true
 max_jobs: 1
-#image: Visual Studio 2015
+image: Visual Studio 2017
 # History plugin requires complete log
 #clone_depth: 5
 branches: