ci: don't use node 8.10, which is broken

nodejs/node#19274

Author: piscisaureus

Diff for: .appveyor.yml

@@ -13,7 +13,7 @@ cache:
 environment:
   matrix:
     - appveyor_build_worker_image: Visual Studio 2015
-      nodejs_version: 8
+      nodejs_version: 8.9
   # disabled to reduce CI wait time:
   # - appveyor_build_worker_image: Visual Studio 2017
   #   nodejs_version: 9