fix #466 development server on OS X

a bit more verbose than I would have liked, and probably doesn't work for Windows.
This works for me:
```
try Some(new File(sys.props.get("java.home").get).getParent)
catch {
  case _: Throwable => None
}
```

(the consensus on the internets is that ${java.home}/../lib/tools.jar is the thing to try)