Include roomToken in GET /rooms/:token: response

As expected and specified in the docs:

* https://docs.services.mozilla.com/loop/apis.html#get-rooms-token
* https://wiki.mozilla.org/Loop/Architecture/Rooms#GET_.2Frooms.2F.7Btoken.7D

Previously, it had been removed:

* https://github.com/mozilla-services/loop-server/commit/f95250a12e4a400db574bdcd1f331022e4471754#diff-2fbbe66e501e7e82cd4c1868a4632212R258