crypto/tls: update generate_cert.go for new time package
crypto/tls: update generate_cert.go for new time package

Fixes issue 2635.

R=golang-dev, iant
CC=golang-dev
http://codereview.appspot.com/5512043

wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old