adding ip6 pg_hba.conf connection line

adding the ip6 equivalent - my first installs on CentOS I missed this, and was baffled why the DB connection was failing - it didn't help that this wasn't in the docs.
Is there any reason why the example can't simply be:

```
host all all localhost md5
```

Since `localhost` exists in `/etc/hosts` for both IPv6 and IPv4 localhost and will map to either depending upon the context.  There's no need to include IP addresses, AFAICT.