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.

lib/prompt_info_functions.zsh

@@ -10,9 +10,15 @@
 # Dummy implementations that return false to prevent command_not_found
 # errors with themes, that implement these functions
 # Real implementations will be used when the respective plugins are loaded
-function chruby_prompt_info hg_prompt_info pyenv_prompt_info \
-  rbenv_prompt_info svn_prompt_info vi_mode_prompt_info \
-  virtualenv_prompt_info jenv_prompt_info {
+function chruby_prompt_info \
+  rbenv_prompt_info \
+  hg_prompt_info \
+  pyenv_prompt_info \
+  svn_prompt_info \
+  vi_mode_prompt_info \
+  virtualenv_prompt_info \
+  jenv_prompt_info \
+{
   return 1
 }
 
@@ -22,10 +28,13 @@ function rvm_prompt_info() {
   [ -f $HOME/.rvm/bin/rvm-prompt ] || return 1
   local rvm_prompt
   rvm_prompt=$($HOME/.rvm/bin/rvm-prompt ${=ZSH_THEME_RVM_PROMPT_OPTIONS} 2>/dev/null)
-  [[ "${rvm_prompt}x" == "x" ]] && return 1
-  echo "${ZSH_THEME_RVM_PROMPT_PREFIX:=(}${rvm_prompt}${ZSH_THEME_RVM_PROMPT_SUFFIX:=)}"
+  [[ -z "${rvm_prompt}" ]] && return 1
+  echo "${ZSH_THEME_RUBY_PROMPT_PREFIX}${rvm_prompt}${ZSH_THEME_RUBY_PROMPT_SUFFIX}"
 }
 
+ZSH_THEME_RVM_PROMPT_OPTIONS="i v g"
+
+
 # use this to enable users to see their ruby version, no matter which
 # version management system they use
 function ruby_prompt_info() {