go.mod: upgrade "go.uber.org/zap" to v1.9.1

Signed-off-by: Gyuho Lee <leegyuho@amazon.com>

go.mod

@@ -47,7 +47,7 @@ require (
 	go.etcd.io/bbolt v1.3.1-etcd.7
 	go.uber.org/atomic v1.3.2 // indirect
 	go.uber.org/multierr v1.1.0 // indirect
-	go.uber.org/zap v1.8.0
+	go.uber.org/zap v1.9.1
 	golang.org/x/crypto v0.0.0-20180608092829-8ac0e0d97ce4
 	golang.org/x/net v0.0.0-20180906233101-161cd47e91fd
 	golang.org/x/time v0.0.0-20180412165947-fbb02b2291d2