move ruby discovery code into a function

When you install a new ruby in a different terminal shell,
chruby does not pick up on the new ruby version.

The solution is to to source chruby.sh again.
Introducing chruby_discover which will allow any shell to pick up on the
new ruby versions

share/chruby/chruby.sh

@@ -1,10 +1,15 @@
 CHRUBY_VERSION="0.3.9"
-RUBIES=()
 
-for dir in "$PREFIX/opt/rubies" "$HOME/.rubies"; do
-	[[ -d "$dir" && -n "$(command ls -A "$dir")" ]] && RUBIES+=("$dir"/*)
-done
-unset dir
+function chruby_discover()
+{
+  RUBIES=()
+  local dir
+	for dir in "$PREFIX/opt/rubies" "$HOME/.rubies"; do
+		[[ -d "$dir" && -n "$(command ls -A "$dir")" ]] && RUBIES+=("$dir"/*)
+	done
+}
+
+chruby_discover
 
 function chruby_reset()
 {