Switch to node version "8" and dont use npm@next to fix the build

appveyor.yml

@@ -1,12 +1,11 @@
 # Test against this version of Node.js
 environment:
-  nodejs_version: "6"
+  nodejs_version: "8"
 
 # Install scripts. (runs after repo cloning)
 install:
   - ps: Install-Product node $env:nodejs_version
   - node --version
-  - npm install -g npm@next
   - npm --version
   - cd build
   - npm install