Switch to using Go modules.
This migrates our vendor.json to use Go 1.11's modules system, and
replaces the vendor folder with the output of go mod vendor.
The vendored code should remain basically the same; I believe some
tree shaking of packages and support scripts/licenses/READMEs/etc.
happened.
This also fixes Travis and our Makefile to no longer use govendor.
I'm working on Stackdriver monitoring in another branch, which required updating `google.golang.org/genproto`, which required updating `google.golang.org/grpc`, which required updating `github.com/golang/protobuf`, and so on. This PR updates all of the Google-provided deps to their latest versions. In addition, there is:
- A change in config.go to reflect an updated type name
- Five files changed by `make fmt`
Tested with `make build`, `make test`, and `make testacc`.
