Add airbrake support.

This leaves the existing email notification of exceptions in place.
That can be removed once we've verified that airbrake is working as
expected.
Remove router-client gem.

This is unused - it was only ever used with the old scala router.  The
new router uses a client in gds-api-adapters
Merge pull request #182 from alphagov/remove_router_client_gem

Remove router-client gem.
Merge pull request #181 from alphagov/add_airbrake

Add airbrake support so we can send errors to errbit.