macOS Sierra: Homebrew: Installation Error: Xcode Outdated Problem

Command (Fixing permission problem first):

$ sudo chown -R $(whoami) /usr/local
Password:

Command:

$ brew install tree


Result:

==> Auto-updated Homebrew!
Updated Homebrew from 9586390 to aaf8c41.
Updated 1 tap (homebrew/core).
==> New Formulae
apache-zeppelin                     libuninameslist                     liquigraph                          micro                               timewarrior                    
==> Updated Formulae
artifactory         ccache              fleetctl            graphicsmagick      imagemagick         libphonenumber      pidcat              syncthing           zsh-completions
aws-sdk-cpp         cli53               fwup                gron                influxdb            libpng ✔            platformio          telegraf        
awscli              cmake               gdm                 gtkextra            jsdoc3              libspiro            pngcrush            todolist        
azure-cli           crystal-lang        git-annex           heroku              lean-cli            mapnik              pypy                yadm            
cask                darcs               go                  hub                 libgetdata          mercurial ✔         sane-backends       youtube-dl      

Warning: You are using OS X 10.12.
We do not provide support for this pre-release version.
You may encounter build failures or other breakages.
Please create pull-requests instead of filing issues.
Error: Your Xcode (7.3.1) is outdated
Please update to Xcode 8.0.
Xcode can be updated from
  https://developer.apple.com/xcode/downloads/
If 8.0 is installed, you may need to:
  sudo xcode-select --switch /path/to/Xcode-beta.app