sameersbn/gitlab expects redis link as redisio

The [sameersbn/gitlab](https://github.com/sameersbn/docker-gitlab) image expects redis to be linked as `redisio` making the [redis-link.conf](https://github.com/SystemDocker/redis/blob/master/dropins/redis-link.conf) dropin not work. The image should be made compatible with this drop-in.