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.