title: How to have yarn not issue a warning for the license field?
body: <p>On running <code>yarn install</code> I see a warning every time that there is no license filed even though I have defined one:</p>
  
  <pre><code>$ jq . package.json 
  {
    "name": "license-example",
    "version": "1.0.0",
    "main": "index.js",
    "license": "UNLICENSED",
    "dependencies": {
      "lodash": "^4.17.4",
      "moment": "^2.18.1"
    }
  }
  </code></pre>
  
  <p>which according to the <a href="https://docs.npmjs.com/files/package.json#license" rel="noreferrer">npm defintion</a> should be valid:</p>
  
  <blockquote>
    <p>Finally, if you do not wish to grant others the right to use a private
    or unpublished package under any terms:</p>
  
  <pre><code>{ "license": "UNLICENSED" }
  </code></pre>
  </blockquote>
  
  <p>Here's the output:</p>
  
  <pre><code>yarn install
  yarn install v0.27.5
  warning ../package.json: No license field
  [1/4] Resolving packages...
  success Already up-to-date.
  Done in 0.09s.
  </code></pre>
  
  <p>My main goal is for that warning to disappear, yet I also don't want to provide an invalid open-source LICENSE to make the warning go away, even if it is an internal project that never will be seen on the outside.</p>
  
  <p>How to mark a yarn project as proprietary without a warning appearing?</p>