title: I met some problems about installing yarn with npm/nvm
body: <p>After i installed nvm, I ran</p>
  
  <pre><code>nvm install 8.15.0
  </code></pre>
  
  <p>then I have node@v8.15.0 installed.</p>
  
  <p>But when I ran </p>
  
  <pre><code>nvm use 8.15.0
  npm install -g yarn
  </code></pre>
  
  <p>I got this message in my terminal:</p>
  
  <pre><code>/Users/bellemere/.nvm/versions/node/v8.9.4/bin/yarn -&gt; /Users/bellemere/.nvm/versions/node/v8.9.4/lib/node_modules/yarn/bin/yarn.js
  /Users/bellemere/.nvm/versions/node/v8.9.4/bin/yarnpkg -&gt; /Users/bellemere/.nvm/versions/node/v8.9.4/lib/node_modules/yarn/bin/yarn.js
  + yarn@1.15.2
  updated 1 package in 2s
  </code></pre>
  
  <p>Shouldn't yarn be installed under v8.15.0?
  Does anyone meet this problem before?</p>