Upgrade Ur/Web version to fix performance problem

I think I found the performance problem I asked about before, and it should be fixed now in Ur/Web itself.  It turned out to be a very expensive security measure that newer GCCs associate with the "%n" printf() specifier!

I'd really appreciate it if you could run a quick sanity benchmarking of the json test with the new Ur/Web version installed.  I verified myself in EC2 that it's now competitive with netty, the champion from Round 8.  Thanks!!