Fix missing IOError rescue on shutdown (celluloid#212)

I'm not entirely sure that this rescue is in the right place but it
does fix the issue. I did try putting it in handle_connection first
but that was ineffective.

lib/reel/server.rb

@@ -36,6 +36,7 @@ def shutdown
 
     def run
       loop { async.handle_connection @server.accept }
+    rescue IOError
     end
 
     def handle_connection(socket)