websocket: drop support for go1.4 or below

Change-Id: I228f1405aac4ed058dafdfd5fc4cc609c56004fa
Reviewed-on: https://go-review.googlesource.com/30897
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
3 files changed