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/mira.zsh-theme

@@ -4,22 +4,13 @@ local return_code="%(?..%{$fg[red]%}%? ↵%{$reset_color%})"
 local user_host='%{$terminfo[bold]$fg[green]%}%n@%m%{$reset_color%}'
 local current_dir='%{$terminfo[bold]$fg[blue]%} %~%{$reset_color%}'
 
-local rvm_ruby=''
-if which rvm-prompt &> /dev/null; then
-  rvm_ruby='%{$fg[red]%}‹$(rvm-prompt i v g)›%{$reset_color%}'
-else
-  if which rbenv &> /dev/null; then
-    rvm_ruby='%{$fg[red]%}‹$(rbenv version | sed -e "s/ (set.*$//")›%{$reset_color%}'
-  fi
-fi
+local nvm_node='%{$fg[green]%}‹node-$(nvm_prompt_info)›%{$reset_color%}'
 
-local nvm_node=''
-nvm_node='%{$fg[green]%}‹node-$(nvm_prompt_info)›%{$reset_color%}'
+local jenv_java='%{$fg[blue]%}‹$(jenv_prompt_info)›%{$reset_color%}'
 
-local jenv_java=''
-jenv_java='%{$fg[blue]%}‹$(jenv_prompt_info)›%{$reset_color%}'
+local git_branch='$(git_prompt_info)'
 
-local git_branch='$(git_prompt_info)%{$reset_color%}'
+local rvm_ruby='$(ruby_prompt_info)'
 
 PROMPT="╭─${user_host} ${current_dir} ${nvm_node} ${rvm_ruby} ${jenv_java} ${git_branch}
 ╰─%B$%b "
@@ -27,3 +18,6 @@ RPS1="${return_code}"
 
 ZSH_THEME_GIT_PROMPT_PREFIX="%{$fg[yellow]%}("
 ZSH_THEME_GIT_PROMPT_SUFFIX=") %{$reset_color%}"
+
+ZSH_THEME_RUBY_PROMPT_PREFIX="%{$fg[red]%}‹"
+ZSH_THEME_RUBY_PROMPT_SUFFIX="›%{$reset_color%}"