Specify what compier versions are supported

[Minoru]

Our builds produce quite a few warnings, which makes it hard to see if new code introduces new ones. I'd prefer the build to be warning-free, and I'm ready to work on a PR that gets us there, but for that, I need to know which GHC versions Hakyll ought to support.

I think we should only support the last three major releases, because that's what GHC developers themselves seem to support.

@jaspervdj, I'd like to know your opinion on this.