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.
```
govendor fetch google.golang.org/api/serviceusage/v1@04bb50b6b83d0e72253821af8cf3252d8e866517
```
#2646 will need to be done after migrating the existing resources off `v1beta1`
