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/grpc/stats/handlers.go

@@ -19,9 +19,8 @@
 package stats
 
 import (
+	"context"
 	"net"
-
-	"golang.org/x/net/context"
 )
 
 // ConnTagInfo defines the relevant information needed by connection context tagger.