sauce-labs: close log writer when done
sauce-labs: close log writer when done

Without this, the "final" log part is never sent, and logs doesn't
aggregate logs correctly.

Acidburn0zzz/worker,solarce/worker,henrikhodne-abandoned/worker,travis-ci/worker,travis-ci/worker,solarce/worker,Acidburn0zzz/worker,travis-ci/worker,solarce/worker,henrikhodne-abandoned/worker,henrikhodne-abandoned/worker,Acidburn0zzz/worker