ipv4: drop support for go1.4 or below

Change-Id: Ifbc3848dcf1f7d43e3d781bcfe24e144ce028c32
Reviewed-on: https://go-review.googlesource.com/30894
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
12 files changed