Replace preview link. (#6369)

The old preview on Skitch is no longer available (closed account?). The new preview is on a free Flickr which might last longer. ?

themes/gallifrey.zsh-theme

-# ZSH Theme - Preview: http://img.skitch.com/20091113-qqtd3j8xinysujg5ugrsbr7x1y.jpg
+# ZSH Theme - Preview: https://flic.kr/p/ZFvivf
 local return_code="%(?..%{$fg[red]%}%? ↵%{$reset_color%})"
 
 PROMPT='%{$fg[green]%}%m%{$reset_color%} %2~ $(git_prompt_info)%{$reset_color%}%B»%b '