chore: use Node.js 6, 8 and 10

.appveyor.yml

@@ -11,6 +11,8 @@ environment:
       platform: x64
     - nodejs_version: "6"
       platform: x86
+    - nodejs_version: "8"
+      platform: x86
     - nodejs_version: "8"
       platform: x64
     - nodejs_version: "10"