Update to use new get-ruby-version function.

Also refactor to pull out common irb arguments.

05_ruby

@@ -1,10 +1,20 @@
-# Convenient.  Also works in Gentoo or Ubuntu
-if [[ -x $(which irb$(get_ruby_version)) ]]; then
-  alias irb='irb1.9.1 --readline -r irb/completion'
+#
+# Set up ruby
+#
+autoload -U get-ruby-version && get-ruby-version
+
+# If you alias ruby to (say) ruby 1.9, this should update irb too.
+# You might want to alias 'gem' here too, if you do, remember to update your path.
+local irb_options="--readline -r irb/completion"
+
+if [[ -x $(which irb$RUBY_VERSION) ]]; then
+  alias irb='irb$RUBY_VERSION $irb_options'
 else
-  alias irb='irb --readline -r irb/completion'
+  alias irb="irb $irb_options"
 fi
+
 export RI='-f ansi --width 70'
+
 if [[ -x `which fri` ]]; then
   alias ri=fri
 fi