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/identity_classic.go

@@ -12,6 +12,10 @@ import (
 	netcontext "golang.org/x/net/context"
 )
 
+func init() {
+	appengineStandard = true
+}
+
 func DefaultVersionHostname(ctx netcontext.Context) string {
 	c := fromContext(ctx)
 	if c == nil {