title: brew update fails with "Permission denied" error
body: <p>I have been trying to upgrade my brew installation on Mac and it always fails with the following error:</p>
  
  <pre><code>Error: An unexpected error occurred during the `brew link` step
  The formula built, but is not symlinked into /usr/local
  Permission denied @ dir_s_mkdir - /usr/local/Frameworks
  Error: Permission denied @ dir_s_mkdir - /usr/local/Frameworks
  </code></pre>
  
  <p>I do not want to run brew as sudo, but is there a way to fix this error? I'm not able to install Python3 which is my ultimate goal. Is there another way to install Pythion3 on Mc other than brew? Is there any resolution for this problem here?</p>
  
  <p>EDIT: I tried to remove python3 and tried to re install it again and this time it fails with some other message:</p>
  
  <pre><code>==&gt; Pouring python-3.6.5.high_sierra.bottle.1.tar.gz
  Error: An unexpected error occurred during the `brew link` step
  The formula built, but is not symlinked into /usr/local
  Permission denied @ dir_s_mkdir - /usr/local/Frameworks
  Error: Permission denied @ dir_s_mkdir - /usr/local/Frameworks
  </code></pre>