Enable NPN

Next Protocol Negotiation extension. See https://technotes.googlecode.com/git/nextprotoneg.html

This should reduce round-trip time (RTT) for clients, thus giving faster load times.

This is essentially a micro-optimisation to try to turn on all the switches that can make things faster.
```
2014/12/01 10:42:27 [emerg] 21382#0: "client_max_body_size" directive is duplicate in /etc/nginx/sites-enabled/admin-beta.development.performance.service.gov.uk.conf:54
```