Update rtax to 0.984

Updates rtax to 0.984 in order to resolve #1012.  I'm sorry that I don't have time to set up a new Qiime installation to run the tests just now.  I'd much appreciate it if you can run the tests, on the theory that it's much easier for you to do it.  If they pass, as I hope and expect, then we're done.  If they fail, then of course I'll go through the exercise to fix it up.  Thanks!