Sticky routing with caching

Essentially,

- Setting a cookie with the specific service version.
- Assigning that cookie value to an 'x-version' request header so that the
  Varnish cache doesn't have to vary on cookie.

This will allow us to use cookies to persist users to certain
applications (eg, so they don't bounce between v1 and v2 each request).

- This is not a general solution, but shows it's trivial to assign a
  cookie to a cache key.
- I think the cookie setting should happen at application level.
Tidy tests
Merge pull request #1 from Financial-Times/sticky-routes

Sticky routes