appveyor: update to later version of tools

After AppVeyor latest changes this needs to be updated also

Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.com>
diff --git a/.appveyor.yml b/.appveyor.yml
index 500d66f..5ea3f7c 100644
--- a/.appveyor.yml
+++ b/.appveyor.yml
@@ -2,7 +2,7 @@
 pull_requests:
   do_not_increment_build_number: true
 skip_tags: true
-image: Previous Visual Studio 2017
+image: Visual Studio 2017
 configuration:
   - Debug
   - Release