title: How to install Homebrew Portable Ruby when your system version is too old
body: <p>I'm trying to install Homebrew on macOS. Once the installation finished, when I typed <code>help</code> I got:</p>
  <pre><code>Already downloaded: /.../Library/Caches/Homebrew/portable-ruby-2.6.3.mavericks.bottle.tar.gz
  Error: Checksum mismatch.
  Expected: ab81211a2052ccaa6d050741c433b728d0641523d8742eef23a5b450811e5104
    Actual: f80fd4cc877a64b0ac1c105fdc212ca6be08bb034118900f2c2f58edcfe0dafd
   Archive: /.../Library/Caches/Homebrew/portable-ruby-2.6.3.mavericks.bottle.tar.gz
  To retry an incomplete download, remove the file above.
  Error: Failed to install Homebrew Portable Ruby (and your system version is too old)!
  </code></pre>