Add rbfu support.

Prompts can now display the currently active Ruby version set by rbfu, a tool similar to RVM and rbenv.
diff --git a/themes/base.theme.bash b/themes/base.theme.bash
index 58efe11..b763121 100644
--- a/themes/base.theme.bash
+++ b/themes/base.theme.bash
@@ -27,6 +27,9 @@
 RBENV_THEME_PROMPT_PREFIX=' |'
 RBENV_THEME_PROMPT_SUFFIX='|'
 
+RBFU_THEME_PROMPT_PREFIX=' |'
+RBFU_THEME_PROMPT_SUFFIX='|'
+
 function scm {
   if [[ -d .git ]]; then SCM=$SCM_GIT
   elif [[ -n "$(git symbolic-ref HEAD 2> /dev/null)" ]]; then SCM=$SCM_GIT
@@ -123,6 +126,12 @@
   fi
 }
 
+function rbfu_version_prompt {
+  if [[ $RBFU_RUBY_VERSION ]]; then
+    echo -e "${RBFU_THEME_PROMPT_PREFIX}${RBFU_RUBY_VERSION}${RBFU_THEME_PROMPT_SUFFIX}"
+  fi
+}
+
 function virtualenv_prompt {
   if which virtualenv &> /dev/null; then
     virtualenv=$([ ! -z "$VIRTUAL_ENV" ] && echo "`basename $VIRTUAL_ENV`") || return
diff --git a/themes/envy/envy.theme.bash b/themes/envy/envy.theme.bash
index 6787434..e9b7081 100644
--- a/themes/envy/envy.theme.bash
+++ b/themes/envy/envy.theme.bash
@@ -10,7 +10,7 @@
 GIT_THEME_PROMPT_SUFFIX="${green}|"
 
 function prompt_command() {
-    PS1="\n${yellow}$(rbenv_version_prompt)$(rvm_version_prompt) ${purple}\h ${reset_color}in ${green}\w\n${bold_cyan}$(scm_char)${green}$(scm_prompt_info) ${green}→${reset_color} "
+    PS1="\n${yellow}$(rbfu_version_prompt)$(rbenv_version_prompt)$(rvm_version_prompt) ${purple}\h ${reset_color}in ${green}\w\n${bold_cyan}$(scm_char)${green}$(scm_prompt_info) ${green}→${reset_color} "
 }
 
 PROMPT_COMMAND=prompt_command;