This change updates the vendored tzdata database to version 2017b
(released 2017-03-20).

The TestFirstZone test (which always uses the vendored database) is
updated to make it work with the new timezones database. (The Tokelau
abbreviation was changed from 'TKT' to the numeric abbreviation in
tzdata-2017a)

Fixes #19376

Change-Id: I0dea93e8703992de5c92c7840b8bacad9d02c50d
Reviewed-on: https://go-review.googlesource.com/44832
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>