title: You don't have write permissions for the /Library/Ruby/Gems/2.3.0 directory
body: <p>When I use command <code>gem install bundler</code> in MacOS 10.13.x, the error is:</p>
  <blockquote>
  <p>You don't have write permissions for the /Library/Ruby/Gems/2.3.0 directory.</p>
  </blockquote>
  <pre><code>$ gem install bundler
  Fetching: bundler-1.16.2.gem (100%)
  ERROR:  While executing gem ... (Gem::FilePermissionError)
  You don't have write permissions for the /Library/Ruby/Gems/2.3.0 directory.
  </code></pre>