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
Remove unnecessary SLICEPATH on CentOS for developers
Merge pull request #970 from sbesson/slicepath_cleanup

Remove unnecessary SLICEPATH on CentOS for developers