Phan Duc Nhat Minh

Fixes #43

Hi @schoeberl, In `arbitrateSimp`, `regEmpty` is updated to true [when (out.ready)](https://github.com/schoeberl/chisel-book/blob/6f8bdcae7e5dd905e9e6c529344717c4f3eec860/src/main/scala/ArbiterTree.scala#L100), but what if `out.ready` is true when `regEmpty` is true? The following test fails while I think it should...