celluloid: "Couldn't cleanly terminate all actors in 10 seconds!" with Spring 1.6.0 and Rails 5.0.0.beta1

Rails 5.0.0.beta1 encourages you to mount the Celluloid-powered ActionCable.server in-process by changing the routes.rb file to uncomment mount ActionCable.server => '/cable'. When you do that, generator commands (that presumably do a fork somewhere) start hanging with the following error message: E, [2015-12-20T08:09:19.218168 #5720] ERROR -- : Couldn't cleanly terminate all actors in 10 seconds!.

You can provoke this with:

rails new terminator
cd terminator
./bin/rails generate scaffold post title:string

If you remove the Spring loader from bin/rails, there is no problem.