postgresql_version is now set in function of ansible_distribution_version

[drosofff]

In group_vars/all postgresql_version is now set as follows

postgresql_version: "{{ '9.3' if ansible_distribution_version <= '15.04' else '9.5' }}"

@mvdbeek @remimarenco with this PR and https://github.com/galaxyproject/ansible-galaxy-extras/pull/137, I think that GKS install correctly in ubuntu 16.04. Also, specifying supervisor_postgres_database_path, supervisor_postgres_database_config and supervisor_postgres_options in group_vars/docker is redundant as set in group_vars/all

[mvdbeek]

This is a nice solution!