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: /Users/username/.rvm/bin/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.
