jenkins: Use the epel-release package provided by CentOS

It is not necessary to download the epel-release RPM from
fedoraproject.org - it is in the CentOS repository.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
jenkins: Don't install remi repository

Signed-off-by: Euan Harris <euan.harris@citrix.com>
README: Update instructions for installing EPEL

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #585 from euanh/jenkins

Jenkins