title: yarn won't update itself
body: <p>I cannot explain why - even if I update it - yarn keeps on staying on this buggy v1.6.0. </p>
  
  <p>I have installed yarn via npm and node is installed via nvm in MacOS.</p>
  
  <pre><code>sscarduzio@hilbert:/me $ yarn -v
  1.6.0
  
  sscarduzio@hilbert:/me $ grep yarn package.json
      "yarn": "^1.7.0"
  
  sscarduzio@hilbert:/me $ which yarn
  /Users/sscarduzio/.nvm/versions/node/v8.11.1/bin/yarn
  
  
  sscarduzio@hilbert:/me $ node -v
  v8.11.1
  
  sscarduzio@hilbert:/me $ npm upgrade --global  yarn
  
  sscarduzio@hilbert:/me $ yarn -v
  1.6.0  &lt;--- WTF??
  </code></pre>
  
  <p>I have ran out of ideas :(</p>