ActionCable isn't *really* a Rails 5 dependency.

application.rb

# Existing applications upgrading to Rails 5 that don't want ActionCable

# config/application.rb
# require 'rails/all'
require "rails"

# same as rails/all.rb, but remove actioncable
require "active_model/railtie"
require "active_job/railtie"
require "active_record/railtie"
require "action_controller/railtie"
require "action_mailer/railtie"
require "action_view/railtie"
# require "action_cable/engine"
require "sprockets/railtie"
require "rails/test_unit/railtie"

Gemfile

# gem 'rails'
gem "activerecord"
gem "actionpack"
gem "actionview"
gem "actionmailer"
gem "activejob"
gem "activesupport"
gem "railties"
gem "sprockets-rails"
gem 'sqlite3'

# If you do this, you don't need to change config/application.rb.
# This has the side effect of not even *installing* ActionCable's gem dependencies.
# via @derekprior

new_application.sh

rails new --skip-action-cable

One disadvantage: you must now manage the version constraints of dependencies like sprockets-rails manually. For example, in a rails 5.0.1 app, use the following constraint.

- gem "sprockets-rails"
+ gem "sprockets-rails", ">= 2.0.0"

Normally, those version constraints would be managed for you by rails.gemspec.

The Gemfile solution will only work if none of your dependencies have the rails gem as a dependency.