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.
Switch to normal zsh %F/%f prompt escapes to avoid "eval" and extra variables.
Wrap whole thing in anonymous function so the existing `local`
statements actually work. Then switch '...' to eager "..." so
stuff actually works with local variables. And local-ize the
remaining variables that are used only in prompt construction.
