Previous commit accidentally omitted "require yum" statement.  Adding it back.
Whether this is the right place to require the yum module is debatable; you'd
have to require it for every single module you installed.  Seems better just
to make the EPEL repository required (install it however you like) for RHEL
systems (don't know about Scientific Linux).

Leaving statement in for now--purpose of pull request is _not_ to change
yum behavior.