Themes: switch to using ruby_prompt_info()

Changes themes displaying RVM or other Ruby version info to use the central
ruby_prompt_info function. This supports more Ruby versioning mechanisms,
reduces copy-and-paste code, and avoids "zsh: no such file or directory: rvm-prompt"
when run on machines that do not have RVM installed.

Changes the prefix/suffix variable names to ZSH_THEME_RUBY_PROMPT_PREFIX and
ZSH_THEME_RUBY_PROMPT_SUFFIX, since they apply to all Ruby versioning mechanisms,
not just RVM.

Allows empty ZSH_THEME_RUBY_PROMPT_PREFIX and ZSH_THEME_RUBY_PROMPT_SUFFIX.

themes/alanpeabody.zsh-theme

@@ -1,17 +1,10 @@
 
 local user='%{$fg[magenta]%}%n@%{$fg[magenta]%}%m%{$reset_color%}'
 local pwd='%{$fg[blue]%}%~%{$reset_color%}'
-local rvm=''
-if which rvm-prompt &> /dev/null; then
-  rvm='%{$fg[green]%}‹$(rvm-prompt i v g)›%{$reset_color%}'
-else
-  if which rbenv &> /dev/null; then
-    rvm='%{$fg[green]%}‹$(rbenv version | sed -e "s/ (set.*$//")›%{$reset_color%}'
-  fi
-fi
 local return_code='%(?..%{$fg[red]%}%? ↵%{$reset_color%})'
 local git_branch='$(git_prompt_status)%{$reset_color%}$(git_prompt_info)%{$reset_color%}'
 
+ZSH_THEME_RVM_PROMPT_OPTIONS="i v g"
 ZSH_THEME_GIT_PROMPT_PREFIX="%{$fg[green]%}"
 ZSH_THEME_GIT_PROMPT_SUFFIX="%{$reset_color%}"
 ZSH_THEME_GIT_PROMPT_DIRTY=""
@@ -24,5 +17,8 @@ ZSH_THEME_GIT_PROMPT_RENAMED="%{$fg[magenta]%} ➜"
 ZSH_THEME_GIT_PROMPT_UNMERGED="%{$fg[yellow]%} ═"
 ZSH_THEME_GIT_PROMPT_UNTRACKED="%{$fg[cyan]%} ✭"
 
+ZSH_THEME_RUBY_PROMPT_PREFIX="%{$fg[green]%}‹"
+ZSH_THEME_RUBY_PROMPT_SUFFIX="›%{$reset_color%}"
+
 PROMPT="${user} ${pwd}$ "
-RPROMPT="${return_code} ${git_branch} ${rvm}"
+RPROMPT="${return_code} ${git_branch} \$(ruby_prompt_info)"