use latest npm on windows, nodejs/node-gyp#972

juliangruber · juliangruber · commit 497be30f319e · 2017-02-09T11:35:14.000+01:00
diff --git a/appveyor.yml b/appveyor.yml
@@ -12,6 +12,7 @@ platform:
 
 install:
   - ps: Install-Product node $env:nodejs_version $env:platform
+  - npm install -g npm@latest
   - npm install
 
 test_script:
