install epel-release package from CentOS Extras repository

detail information:
[CentOS-announce] The epel-release package is now available via Extras
http://lists.centos.org/pipermail/centos-announce/2014-September/020526.html
this is assuming that this code will run only on current versions of CentOS and not older versions (which is not the case). 

A change like this would break for older distros (hence the check for `distro.release`)