chore: put appveyor back on node 12, node 13 probably wasn't necessary

.appveyor.yml

@@ -4,7 +4,7 @@ environment:
   matrix:
     # - nodejs_version: "8" 
     # - nodejs_version: "10"
-    - nodejs_version: "13"
+    - nodejs_version: "12"
 platform:
   # - x86
   - x64