ci: use node 8.9.4 (#571)

Node 8.9.4 contain a few security update and use npm 5.6.0 so not need to
install npm in the build process. The build time will a few sec sorter

.nvmrc

@@ -1 +1 @@
-8.9.1
+8.9.4

.travis.yml

@@ -45,7 +45,6 @@ addons:
 
 install:
   - nvm install
-  - npm i -g npm@5.6.0
   - npm install
 
 before_script:

appveyor.yml

@@ -18,7 +18,6 @@ shallow_clone: true
 
 install:
   - ps: Install-Product node $env:nodejs_version
-  - npm i -g npm@5.6.0
   - choco install chromium
   - set CI=true
   - set PATH=%APPDATA%\npm;%PATH%