Bump peer dependency to Hapi >= 6

Yesterday I thought I'd convert our *Hapi 4.1* app (held there by *Travelogue*) over to *Bell*. A glance at the *peerDependencies* made me think this was a good idea. *peerDependencies* handling in *npm* isn't solid, but *Bell* definitely needs *Hapi >= 6* because of `server.location()` in `oauth.js`. 

This is just a proposal that the `README.md` or `package.json` indicate that.

Thanks for spearheading this, @hueniverse.It solves a lot of annoyances we've had with OAuth frameworks.