Fix location of postgres persistent data in readme
Fix location of postgres persistent data in readme

In both 9.3 and 9.4 base images it's `/var/lib/postgresql/data` that is the volume that needs to be mounted. Mounting `/var/lib/postgresql` causes data loss when container stops.

https://github.com/docker-library/postgres/blob/master/9.3/Dockerfile#L43
https://github.com/docker-library/postgres/blob/master/9.4/Dockerfile#L43
azukiapp/docker-postgres,azukiapp/docker-postgres