(RE-813) Update to support EL7 & FC20

As per RE-813, this PR adds support for EL7 and FC20, while removing support for FC17 & FC18. It also moves variable assignment higher up in the code, for aesthetic reasons.

![lipstickpig](https://f.cloud.github.com/assets/344926/1969182/ab616cbc-82ee-11e3-9912-efedb8c5781a.jpeg)