What
===
Add `go.mod` and `go.sum`, using `Go 1.11`.
Why
===
Now that golang/go#24301 has been accepted, lets start using go.mod files
alongside Godep, and keep the two in sync.
Notes
===
There are no changes required to testify to support go.mod. The files were
generated by running `go build` and `go mod tidy.
Merging
===
This PR is intended to be merged after #659 which adds Go1.11 to the list of
supported builds.
