I just updated a project to 5.1 yesterday and noticed that Queue::marshal() has been deprecated: Iron.io "push queues" have been deprecated in favor of typical Iron.io queues and queue listeners. Okay, great - so how exactly do I use "typical iron.io queues?" The project is configured for Iron push queues.
I cannot seem to find anything in the documentation regarding Iron apart from how to install it.
Most likely, you forgot to update ironmq version after switching to Laravel 5.1. Only IronMQ v4 is compatible with Laravel 5.1. Specify "4.*" as IronMQ version in composer.json.