gobuilder: send commit time in RFC3339 format
gobuilder: send commit time in RFC3339 format

R=golang-dev, gri
CC=golang-dev
http://codereview.appspot.com/5701063

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang