all: add a go.mod file

This change adds a go.mod file to this repo, following the
requirements stated in bcmills's comment here:
https://golang.org/issue/28136#issuecomment-462971974. It's
important to note that we will not be
adding versions to the repo for now.

This change does not add a go.sum file because this repo has no
dependencies on other repos.

The change was generated by running "go mod init" and "go mod tidy" using Go
1.11.5 in the top-level directory of the repository.

Updates golang/go#28136

Change-Id: If06ef87228cd9c8520de9aa457a97947caaaf53f
Reviewed-on: https://go-review.googlesource.com/c/time/+/162824
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>