adding ip6 connection line to pg_hba.conf

I was following the docs on my original CentOS installs, where the IP6 local connection was being tried and failing, since this was missing from 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.