Update centos.md

Added the flag '--enablerepo=epel' within the CentOS 6 instructions, as I had problems installing on a CentOS 6.5 box. Although the EPEL repository had been successfully added to the list, I had to explicitly include it in the 'yum' command in order to search it.

Signed-off-by: Chris Spiliotopoulos <chrysanthos.spiliotopoulos@gmail.com>