ipv6: drop support for go1.4 or below

Change-Id: I95dfb05e5c958c2949059c4230c023968a442b50
Reviewed-on: https://go-review.googlesource.com/30895
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
13 files changed