dl: add go1.13.6 and go1.12.15

Change-Id: Ibfe011340d2b5605e9c79f711dc45c3685f694fc
Reviewed-on: https://go-review.googlesource.com/c/dl/+/214197
Run-TryBot: Carlos Amedee <carlos@golang.org>
Run-TryBot: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
2 files changed