ci: add cache and match with nvmrc on appveyor

Author: charpeni

.appveyor.yml

@@ -1,13 +1,12 @@
 environment:
   matrix:
-  - nodejs_version: "6.9"
+    - nodejs_version: "6"
 
 matrix:
   fast_finish: true
 
 install:
   - ps: Install-Product node $env:nodejs_version
-  - npm install -g yarn
   - yarn install
 
 test_script:
@@ -17,3 +16,7 @@ test_script:
   - node tests\run_e2e.js
 
 build: off
+
+cache:
+  - node_modules
+  - "%LOCALAPPDATA%/Yarn"