Update appveyor setup

- turning off node_modules cache (causes errors with node stable)
- updating target version range ⬆️

Author: vladimyr

appveyor.yml

@@ -1,9 +1,9 @@
 # Test against this version of Node.js
 environment:
   matrix:
-    - nodejs_version: "6"
     - nodejs_version: "7"
     - nodejs_version: "8"
+    - nodejs_version: "9"
     - nodejs_version: Stable
 
 # Install scripts. (runs after repo cloning)
@@ -19,8 +19,5 @@ test_script:
   - npm --version
   - npm run test:all
 
-cache:
-  - node_modules
-
 # Don't actually build.
 build: off