chore: enhance appveyor build

Author: buehler

.appveyor.yml

@@ -1,21 +1,17 @@
-# Test against the latest version of this Node.js version
+version: "{build} - {branch}"
+skip_tags: true
+
 environment:
-  nodejs_version: "8"
+  matrix:
+    - nodejs_version: "8"
+    - nodejs_version: "7"
+    - nodejs_version: "6"
 
-# Install scripts. (runs after repo cloning)
 install:
-  # Get the latest stable version of Node.js or io.js
   - ps: Install-Product node $env:nodejs_version
-  # install modules
   - npm install
 
-# Post-install test scripts.
 test_script:
-  # Output useful info for debugging.
-  - node --version
-  - npm --version
-  # run tests
   - npm test
 
-# Don't actually build.
-build: off
+build: off