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/contrib.go.opencensus.io/exporter/ocagent/nodeinfo.go

@@ -21,7 +21,12 @@ import (
 	"go.opencensus.io"
 )
 
-func createNodeInfo(nodeName string) *commonpb.Node {
+// NodeWithStartTime creates a node using nodeName and derives:
+//  Hostname from the environment
+//  Pid from the current process
+//  StartTimestamp from the start time of this process
+//  Language and library information.
+func NodeWithStartTime(nodeName string) *commonpb.Node {
 	return &commonpb.Node{
 		Identifier: &commonpb.ProcessIdentifier{
 			HostName:       os.Getenv("HOSTNAME"),