use_package_json is not idempotent

[fnoop]

Affected Puppet, Ruby, OS and module versions/distributions

• Puppet: 5.2.0
• Ruby: 2.3.1p112
• Distribution: Ubuntu 16.04
• Module version: 5.0.0

How to reproduce (e.g Puppet code you use)

nodejs::npm { 'npm-maverick-web':
      user             => 'mav',
      home_dir         => '/srv/maverick',
      ensure           => 'present',
      target           => '/srv/maverick/code/maverick-web',
      use_package_json => true,
    } 

What are you seeing

Notice: /Stage[main]/Maverick_web::Maverick_web/Nodejs::Npm[npm-maverick-web]/Exec[npm_install_npm-maverick-web]/returns: executed successfully
This is seen every run

What behaviour did you expect instead

It should only run once