misc/dashboard/builder: fix bug that caused old revisions to be rebuilt
misc/dashboard/builder: fix bug that caused old revisions to be rebuilt

R=rsc, r2
CC=golang-dev
http://codereview.appspot.com/2213044
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go