Install latest EPEL release

Use a little trick cribbed from [1] to install the latest EPEL
release, rather than hard-coding versions

[1] http://stackoverflow.com/questions/14016286/how-to-programmatically-install-the-latest-epel-release-rpm-without-knowing-its

Change-Id: Ib89d73d669efe2e2d61fc0b12b46395fce113386