Update to use go modules (#671)

This includes updates to CI config, vendor files, etc.
I've hard-coded the k8s depedency at 1.13.4 to keep it inline with what
we currently have and to make sure a another run of `go mod tidy`
doesn't accidentally update it to an unexpected version.

Thanks to hectorj2f for carrying this along.

vendor/google.golang.org/appengine/internal/main.go

@@ -11,5 +11,6 @@ import (
 )
 
 func Main() {
+	MainPath = ""
 	appengine_internal.Main()
 }