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/macovsky-ruby.zsh-theme

@@ -1,15 +0,0 @@
-# ZSH Theme - Preview: https://i.gyazo.com/8becc8a7ed5ab54a0262a470555c3eed.png
-local return_code="%(?..%{$fg[red]%}%? ↵%{$reset_color%})"
-
-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$%b '
-else
-  if which rbenv &> /dev/null; then
-    PROMPT='%{$fg[green]%}%~%{$reset_color%} %{$fg[red]%}‹$(rbenv version | sed -e "s/ (set.*$//")› %{$reset_color%} $(git_prompt_info)%{$reset_color%}%B$%b '
-  fi
-fi
-
-RPS1="${return_code}"
-
-ZSH_THEME_GIT_PROMPT_PREFIX="%{$fg[yellow]%}‹"
-ZSH_THEME_GIT_PROMPT_SUFFIX="› %{$reset_color%}"

themes/macovsky-ruby.zsh-theme

@@ -0,0 +1 @@
+macovsky.zsh-theme