Only test against supported Node versions

Electron Packager only supports Node versions which are supported the
Node.js team, when testing via CI

See: https://github.com/nodejs/Release

.travis.yml

@@ -7,10 +7,9 @@ osx_image: xcode8
 language: node_js
 node_js:
 - '4'
-- '5'
 - '6'
-- '7'
 - '8'
+- '9'
 cache:
 - apt: true
 - directories:

test/ci/appveyor.yml

@@ -4,10 +4,9 @@ platform:
 environment:
   matrix:
   - nodejs_version: "4"
-  - nodejs_version: "5"
   - nodejs_version: "6"
-  - nodejs_version: "7"
   - nodejs_version: "8"
+  - nodejs_version: "9"
 cache:
 - '%APPDATA%\npm-cache'
 - '%USERPROFILE%\.electron'