actually use 16.04 on 16.04 builds (swift-server#159)

motivation: 16.04 docker-compose setup was pulling 18.04 by mistake changes: use xenial instead of bionic

Author: tomerd

docker/docker-compose.1604.51.yaml

@@ -6,7 +6,7 @@ services:
 image: async-http-client:16.04-5.1
 build:
 args:
-ubuntu_version: "bionic"
+ubuntu_version: "xenial"
 swift_version: "5.1.3"
 
 test: