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/go.opencensus.io/plugin/ochttp/span_annotating_client_trace.go

@@ -29,6 +29,8 @@ type spanAnnotator struct {
 
 // TODO: Remove NewSpanAnnotator at the next release.
 
+// NewSpanAnnotator returns a httptrace.ClientTrace which annotates
+// all emitted httptrace events on the provided Span.
 // Deprecated: Use NewSpanAnnotatingClientTrace instead
 func NewSpanAnnotator(r *http.Request, s *trace.Span) *httptrace.ClientTrace {
 	return NewSpanAnnotatingClientTrace(r, s)