Node >= v4 Required + No `new`

While we've never been explicit in our support of a specific Node environment, this release is the first to state only v4 and newer is supported.

Additionally, we've never shown using this module with the new keyword, but it has technically been possible. That is no longer an option. All instances are created just by invoking the function:

var googleAutoAuth = require('google-auto-auth')

// No:
var authClient = new googleAutoAuth({...})

// Yes:
var authClient = googleAutoAuth({...})