Replace HTTPS mirrorlink to epel repo with HTTP via sed

The epel repo is served with SSL 3.0 and breaks things badly on
initial provision in the VM because of the image we're using.

For now, we can use `sed` to replace the scheme of the URL.
Merge pull request #212 from washingtonstateuniversity/epel-force

[PR] Replace HTTPS mirrorlink to epel repo with HTTP via sed