Centos/RHEL installation guide update (#288)

Update CentOS6 install instructions for GitLab 8.9

install/centos/PUIAS_6_computational.repo

-[PUIAS_6_computational]
-name=PUIAS computational Base $releasever - $basearch
-mirrorlist=http://puias.math.ias.edu/data/puias/computational/$releasever/$basearch/mirrorlist
-#baseurl=http://puias.math.ias.edu/data/puias/computational/$releasever/$basearch
-enabled=1
-gpgcheck=1
-gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-puias