acme, acme/autocert: switch to Go 1.7+ context package
The standard library context package has been available since Go 1.7,
and the Go build dashboard (build.golang.org) only tests master and
the past two releases.
Also, the acme package makes no backwards compatibility guarantees,
not that I expect this to influence many people.
Change-Id: Ia1a294212e1e531f28f53cd954d4743a64611cab
Reviewed-on: https://go-review.googlesource.com/39272
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Alex Vaghin <ddos@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
8 files changed
