Add centOS support

[noelmcloughlin]

There is later version of git available for EPEL here: https://ius.io/

# yum install https://centos7.iuscommunity.org/ius-release.rpm && yum swap git git2u

[noelmcloughlin]

The git supplied in centos7 is old so can be replaced like this.

yum install epel-release
yum remove git
rpm -U https://centos7.iuscommunity.org/ius-release.rpm
yum install git2u