update indirect dependency in go.mod

Author: Roberto Sora

go.mod

@@ -50,6 +50,7 @@ require (
 	golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2
 	golang.org/x/net v0.0.0-20190311183353-d8887717615a
 	golang.org/x/text v0.3.0
+	google.golang.org/appengine v1.4.0 // indirect
 	google.golang.org/genproto v0.0.0-20190327125643-d831d65fe17d // indirect
 	google.golang.org/grpc v1.21.1
 	gopkg.in/airbrake/gobrake.v2 v2.0.9 // indirect