title: Error of "error: could not lock config file .git/config: Permission denied" occurs while installing Carthage
body: <p>I want to install carthage on my Mac OS using <code>brew install carthage</code> command. However, I get the following error:</p>
  
  <pre><code>touch: /usr/local/Homebrew/.git/FETCH_HEAD: Permission denied
  touch: /usr/local/Homebrew/Library/Taps/caskroom/homebrew-cask/.git/FETCH_HEAD: Permission denied
  touch: /usr/local/Homebrew/Library/Taps/dart-lang/homebrew-dart/.git/FETCH_HEAD: Permission denied
  touch: /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/.git/FETCH_HEAD: Permission denied
  fatal: Unable to create '/usr/local/Homebrew/.git/index.lock': Permission denied
  error: could not lock config file .git/config: Permission denied
  Warning: carthage 0.26.2 is already installed, it's just not linked.
  You can use `brew link carthage` to link this version.
  </code></pre>
  
  <p>I also get the following error when I used <code>sudo brew install carthage</code>:</p>
  
  <pre><code>Error: Running Homebrew as root is extremely dangerous and no longer supported.
  As Homebrew does not drop privileges on installation you would be giving all
  build scripts full access to your system.
  </code></pre>
  
  <p>Can you let me know what is the problem. Thanks in advance. </p>