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/connection.go

@@ -27,7 +27,10 @@ const (
 
 func (ae *Exporter) setStateDisconnected() {
 	atomic.StoreInt32(&ae.connectionState, sDisconnected)
-	ae.disconnectedCh <- true
+	select {
+	case ae.disconnectedCh <- true:
+	default:
+	}
 }
 
 func (ae *Exporter) setStateConnected() {