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/wedisagree.zsh-theme

@@ -28,7 +28,7 @@ PROMPT='%{$fg[magenta]%}[%c] %{$reset_color%}'
 RPROMPT='${time} %{$fg[magenta]%}$(git_prompt_info)%{$reset_color%}$(git_prompt_status)%{$reset_color%}$(git_prompt_ahead)%{$reset_color%}'
 
 # Add this at the start of RPROMPT to include rvm info showing ruby-version@gemset-name
-# %{$fg[yellow]%}$(~/.rvm/bin/rvm-prompt)%{$reset_color%} 
+# $(ruby_prompt_info) 
 
 # local time, color coded by last return code
 time_enabled="%(?.%{$fg[green]%}.%{$fg[red]%})%*%{$reset_color%}"
@@ -48,6 +48,9 @@ ZSH_THEME_GIT_PROMPT_RENAMED="%{$fg[blue]%} ➜" # ⓡ ⑄
 ZSH_THEME_GIT_PROMPT_UNMERGED="%{$fg[magenta]%} ♒" # ⓤ ⑊
 ZSH_THEME_GIT_PROMPT_AHEAD="%{$fg[blue]%} 𝝙"
 
+ZSH_THEME_RUBY_PROMPT_PREFIX="%{$fg[yellow]%}"
+ZSH_THEME_RUBY_PROMPT_SUFFIX="%{$reset_color%}"
+
 # More symbols to choose from:
 # ☀ ✹ ☄ ♆ ♀ ♁ ♐ ♇ ♈ ♉ ♚ ♛ ♜ ♝ ♞ ♟ ♠ ♣ ⚢ ⚲ ⚳ ⚴ ⚥ ⚤ ⚦ ⚒ ⚑ ⚐ ♺ ♻ ♼ ☰ ☱ ☲ ☳ ☴ ☵ ☶ ☷
 # ✡ ✔ ✖ ✚ ✱ ✤ ✦ ❤ ➜ ➟ ➼ ✂ ✎ ✐ ⨀ ⨁ ⨂ ⨍ ⨎ ⨏ ⨷ ⩚ ⩛ ⩡ ⩱ ⩲ ⩵  ⩶ ⨠