ci(appveyor): Install npm@5

Author: joshwiens

appveyor.yml

@@ -20,6 +20,7 @@ matrix:
   fast_finish: true
 install:
   - ps: Install-Product node $env:nodejs_version x64
+  - npm i -g npm@^5.0.0
   - npm install
 before_test:
   - cmd: npm install webpack@^%webpack_version%