Update server-postgresql.txt

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.
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.
Merge pull request #961 from kennethgillen/patch-2

adding ip6 pg_hba.conf connection line
Merge pull request #962 from kennethgillen/patch-3

adding ip6 connection line to pg_hba.conf
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