Unpin rake, let it be a dependency of rails

Passenger is no longer used, so no need to pin rake to an old version.
Merge pull request #186 from alphagov/upgrade-rake

Update rake to latest
Better errors for SSOPushClient on Errbit
Fine-tune sidekiq worker config to suit Signon
Merge pull request #187 from alphagov/sso-push-client-error-handling

Sso push client error handling