handle case where ~/.rvm/bin/rvm-prompt is not in path, so "which" can't find it

themes/macovsky.zsh-theme

@@ -1,7 +1,7 @@
 # ZSH Theme - Preview: http://gyazo.com/8becc8a7ed5ab54a0262a470555c3eed.png
 local return_code="%(?..%{$fg[red]%}%? ↵%{$reset_color%})"
 
-if which rvm-prompt &> /dev/null; then
+if [ -e ~/.rvm/bin/rvm-prompt ]; then
   PROMPT='%{$fg[green]%}%~%{$reset_color%} %{$fg[red]%}‹$(~/.rvm/bin/rvm-prompt i v)› %{$reset_color%} $(git_prompt_info)%{$reset_color%}%B$%b '
 else
   if which rbenv &> /dev/null; then