Add redis-sentinel to Codis bin dir (#1339)

[LGTM] Thanks !!

Author: anywhy

Makefile

@@ -33,6 +33,7 @@ codis-server:
 	@cp -f extern/redis-3.2.8/src/redis-server  bin/codis-server
 	@cp -f extern/redis-3.2.8/src/redis-benchmark bin/
 	@cp -f extern/redis-3.2.8/src/redis-cli bin/
+	@cp -f extern/redis-3.2.8/src/redis-sentinel bin/
 	@cp -f extern/redis-3.2.8/redis.conf config/
 	@sed -e "s/^sentinel/# sentinel/g" extern/redis-3.2.8/sentinel.conf > config/sentinel.conf