Support hitting checkpoint urls that use SNI - like dashboard.ophan.co.uk

Prout was unable to hit https://dashboard.ophan.co.uk/login because that
server uses Server Name Indication (SNI) - and Play's WS lib is currently
based on a (pre 1.9.0) version of async-http-client that doesn't support
SNI. The latest version of AHC supports SNI but unfortunately it's not
binary compatible with the earlier version currently used by Play.

OkHttp is an HTTP library that _does_ have SNI support:

http://square.github.io/okhttp/

    "OkHttp initiates new connections with modern TLS features (SNI, ALPN),
    and falls back to SSLv3 if the handshake fails."

Testing whether a server is using SNI:

http://blog.chrismeller.com/testing-sni-certificates-with-openssl

```
$ openssl s_client -servername dashboard.ophan.co.uk -connect dashboard.ophan.co.uk:443 -prexit
$ openssl s_client -connect dashboard.ophan.co.uk:443 -prexit
```

This should make https://github.com/guardian/ophan/pull/729 work now!
I've added a functional test for hitting the Ophan endpoint.