Change engine>node{4 => 6} for template

[sudo-suhas]

The template is no longer compatible with node 4. If I create a project from the development branch and try to do npm run dev, I see this:

 ERROR  Failed to compile with 2 errors                                                                                                                       8:32:08 AM

 error  in ./src/App.vue
Syntax Error: Unexpected token {
 @ ./src/main.js 2:0-24
 @ multi ./build/dev-client ./src/main.js

 error  in ./src/components/Hello.vue
Syntax Error: Unexpected token {
 @ ./src/router/index.js 3:0-39
 @ ./src/main.js
 @ multi ./build/dev-client ./src/main.js

> Listening at http://localhost:8080

This updates the engines>node field in the template package.json to indicate the minimum required version.

Note: This PR is a clone of vuejs-templates/webpack#1206(merged)

[jeffposnick]

Thanks!