Update Plek and GDS-API-Adapters

These are required for the new Router functionality to work.
Use new Router API in Rake task

- `register` has now changed to work
- `register_application` has been renamed to `register_backend` to match
  new router nomenclature
- Added new `register_guidance` task to manually register all existing
  DetailedGuide objects with the router.
Add router registration service listeners

When publishing or force publishing, a DetailedGuide should be indexed
in the router. When a DetailedGuide is unpublished, it should be removed
from the router.
Handle case where route doesn't exist

Deleting a route from the router-api returns a 404 error if the route
doesn't exist. Because the end result is the same (the route is
unreachable) whether this succeeds or not we should handle and swallow
the exception.
We don't need full objects for some of the objects
Convert Router helpers to Sidekiq workers

This should make the callbacks faster, as well as providing better
separation of concerns within the application.
Disable the Router API client during Cucumber
Monkeypatch router in cucumber tests
Simplify ServiceListeners::RouterRegistrar
Merge pull request #1005 from alphagov/router-register-on-publish

https://www.pivotaltracker.com/s/projects/367813/stories/60246024

Router register on publish
Merge commit '44df635712618238b11b16818b49dd640935606f' into HEAD