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/amuse.zsh-theme

@@ -1,17 +1,5 @@
 # vim:ft=zsh ts=2 sw=2 sts=2
 
-rvm_current() {
-  rvm current 2>/dev/null
-}
-
-rbenv_version() {
-  rbenv version 2>/dev/null | awk '{print $1}'
-}
-
-PROMPT='
-%{$fg_bold[green]%}%~%{$reset_color%}$(git_prompt_info) ⌚ %{$fg_bold[red]%}%*%{$reset_color%}
-$ '
-
 # Must use Powerline font, for \uE0A0 to render.
 ZSH_THEME_GIT_PROMPT_PREFIX=" on %{$fg[magenta]%}\uE0A0 "
 ZSH_THEME_GIT_PROMPT_SUFFIX="%{$reset_color%}"
@@ -19,11 +7,12 @@ ZSH_THEME_GIT_PROMPT_DIRTY="%{$fg[red]%}!"
 ZSH_THEME_GIT_PROMPT_UNTRACKED="%{$fg[green]%}?"
 ZSH_THEME_GIT_PROMPT_CLEAN=""
 
-if [ -e ~/.rvm/bin/rvm-prompt ]; then
-  RPROMPT='%{$fg_bold[red]%}‹$(rvm_current)›%{$reset_color%}'
-else
-  if which rbenv &> /dev/null; then
-    RPROMPT='%{$fg_bold[red]%}$(rbenv_version)%{$reset_color%}'
-  fi
-fi
+ZSH_THEME_RUBY_PROMPT_PREFIX="%{$fg_bold[red]%}‹"
+ZSH_THEME_RUBY_PROMPT_SUFFIX="›%{$reset_color%}"
+
+PROMPT='
+%{$fg_bold[green]%}%~%{$reset_color%}$(git_prompt_info) ⌚ %{$fg_bold[red]%}%*%{$reset_color%}
+$ '
+
+RPROMPT='$(ruby_prompt_info)'