Fix bug in worker clean-up in UI

Introduced in d5a96fec. This should be picked into 0.8 and 0.9 as well.
Merge pull request #496 from pwendell/master

Fix bug in worker clean-up in UI

Introduced in d5a96fec (/cc @aarondav).

This should be picked into 0.8 and 0.9 as well. The bug causes old (zombie) workers on a node to not disappear immediately from the UI when a new one registers.