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/golang.org/x/net/websocket/websocket.go

@@ -241,7 +241,10 @@ func (ws *Conn) Close() error {
 	return err1
 }
 
+// IsClientConn reports whether ws is a client-side connection.
 func (ws *Conn) IsClientConn() bool { return ws.request == nil }
+
+// IsServerConn reports whether ws is a server-side connection.
 func (ws *Conn) IsServerConn() bool { return ws.request != nil }
 
 // LocalAddr returns the WebSocket Origin for the connection for client, or