Add Host header to nginx conf

The Host header wasn't being picked up by the play app.
We need to set it explicitly in the nginx config.

This is particularly useful in the `routes` reflection and [abosulteUrl lookup](https://github.com/playframework/playframework/blob/master/framework/src/play/src/main/java/play/mvc/Call.java#L45)

![Header In!](http://a.gifb.in/112011/1320956249_long_header_goal__58_m.gif)