add upgrade for zsh?

[pjungermann]

brew install zsh && brew link --overwrite zsh

the link step is necessary to fix a conflict

Error: The `brew link` step did not complete successfully
The formula built, but is not symlinked into /usr/local
Could not symlink bin/zsh
Target /usr/local/bin/zsh
already exists. You may want to remove it:
  rm '/usr/local/bin/zsh'

To force the link and overwrite all conflicting files:
  brew link --overwrite zsh

To list all files that would be deleted:
  brew link --overwrite --dry-run zsh

Possible conflicting files are:
/usr/local/bin/zsh -> /bin/zsh