Update Zsh completions config for Homebrew

https://github.com/zsh-users/zsh-completions

Output after `brew upgrade`:

```
==> Caveats
==> zsh-completions
To activate these completions, add the following to your .zshrc:

  if type brew &>/dev/null; then
    FPATH=$(brew --prefix)/share/zsh-completions:$FPATH

    autoload -Uz compinit
    compinit
  fi

You may also need to force rebuild `zcompdump`:

  rm -f ~/.zcompdump; compinit

Additionally, if you receive "zsh compinit: insecure directories"
warnings when attempting to load these completions, you may need to run
this:

  chmod go-w '/usr/local/share'
```

.zshrc

@@ -43,5 +43,9 @@ bindkey '^N' kill-buffer          # ctrl+n     delete all lines
 bindkey '^_' undo                 # ctrl+/     undo
 bindkey '^\' redo                 # ctrl+\     redo
 ### ---------------------------- Integrations ----------------------------- ###
-fpath+=~/.zfunc:/usr/local/share/zsh-completions
+if type brew &>/dev/null; then
+  FPATH=$(brew --prefix)/share/zsh-completions:$FPATH
+  autoload -Uz compinit
+  compinit
+fi
 source /usr/local/share/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh