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.
The default `pg_hba.conf` created by initdb contains (postgresql93-9.3.5-1PGDG.rhel6):
```
# TYPE  DATABASE        USER            ADDRESS                 METHOD

# "local" is for Unix domain socket connections only
local   all             all                                     peer
# IPv4 local connections:
host    all             all             127.0.0.1/32            ident
# IPv6 local connections:
host    all             all             ::1/128                 ident
# Allow replication connections from localhost, by a user with the
# replication privilege.
#local   replication     postgres                                peer
#host    replication     postgres        127.0.0.1/32            ident
#host    replication     postgres        ::1/128                 ident
```
so it makes sense to keep the same format.