Fixing glitchy kill ring behavior in irb

See https://github.com/ruby/irb/issues/85 for details

lib/reline/line_editor.rb

@@ -183,7 +183,7 @@ class Reline::LineEditor
     @cleared = false
     @rerender_all = false
     @history_pointer = nil
-    @kill_ring = Reline::KillRing.new
+    @kill_ring ||= Reline::KillRing.new
     @vi_clipboard = ''
     @vi_arg = nil
     @waiting_proc = nil