build: upgrade node to 8.8.1 (#471)

* build: upgrade to nodejs 8.8.1 and npm 5.5.1

* ci: upgrade the npm to the latest version in ci files

* build: remove duplication from clean script

appveyor.yml

@@ -18,6 +18,7 @@ shallow_clone: true
 
 install:
   - ps: Install-Product node $env:nodejs_version
+  - npm i -g npm@latest
   - choco install chromium
   - set CI=true
   - set PATH=%APPDATA%\npm;%PATH%