Work around a glitch in setuptools

The `--root` option doesn't seem to work right here. Oh well, prefixing our prefix seems to work.

Sorry I missed this. It seems my build server got confused when you pushed 1.1.0-3 while I was testing this fix. I've thoroughly tested this now, and it should work right.

Thanks,

--scott

(again, specifically tagging @k-okada because this is a release repo)