After the commit for #3763 went in, my production build DEB instance shows:

uninitialized constant ActionView::Template::Handlers::Rabl (NameError)
  /usr/share/foreman/config/initializers/rabl_init.rb:73:in `<top (required)>'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/engine.rb:655:in `block in load_config_initializer'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/notifications.rb:168:in `instrument'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/engine.rb:654:in `load_config_initializer'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/engine.rb:612:in `block (2 levels) in <class:Engine>'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/engine.rb:611:in `each'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/engine.rb:611:in `block in <class:Engine>'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/initializable.rb:30:in `instance_exec'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/initializable.rb:30:in `run'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/initializable.rb:59:in `block in run_initializers'
  /usr/lib/ruby/2.3.0/tsort.rb:228:in `block in tsort_each'
  /usr/lib/ruby/2.3.0/tsort.rb:350:in `block (2 levels) in each_strongly_connected_component'
  /usr/lib/ruby/2.3.0/tsort.rb:422:in `block (2 levels) in each_strongly_connected_component_from'
  /usr/lib/ruby/2.3.0/tsort.rb:431:in `each_strongly_connected_component_from'
  /usr/lib/ruby/2.3.0/tsort.rb:421:in `block in each_strongly_connected_component_from'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/initializable.rb:48:in `each'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/initializable.rb:48:in `tsort_each_child'
  /usr/lib/ruby/2.3.0/tsort.rb:415:in `call'
  /usr/lib/ruby/2.3.0/tsort.rb:415:in `each_strongly_connected_component_from'
  /usr/lib/ruby/2.3.0/tsort.rb:349:in `block in each_strongly_connected_component'
  /usr/lib/ruby/2.3.0/tsort.rb:347:in `each'
  /usr/lib/ruby/2.3.0/tsort.rb:347:in `call'
  /usr/lib/ruby/2.3.0/tsort.rb:347:in `each_strongly_connected_component'
  /usr/lib/ruby/2.3.0/tsort.rb:226:in `tsort_each'
  /usr/lib/ruby/2.3.0/tsort.rb:205:in `tsort_each'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/initializable.rb:58:in `run_initializers'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/application.rb:353:in `initialize!'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/railtie.rb:185:in `public_send'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/railtie.rb:185:in `method_missing'
  /usr/share/foreman/config/environment.rb:5:in `<top (required)>'
  config.ru:3:in `require'
  config.ru:3:in `block in <main>'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/rack-2.0.3/lib/rack/builder.rb:55:in `instance_eval'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/rack-2.0.3/lib/rack/builder.rb:55:in `initialize'
  config.ru:1:in `new'
  config.ru:1:in `<main>'
  /usr/share/passenger/helper-scripts/rack-preloader.rb:110:in `eval'
  /usr/share/passenger/helper-scripts/rack-preloader.rb:110:in `preload_app'
  /usr/share/passenger/helper-scripts/rack-preloader.rb:156:in `<module:App>'
  /usr/share/passenger/helper-scripts/rack-preloader.rb:30:in `<module:PhusionPassenger>'
  /usr/share/passenger/helper-scripts/rack-preloader.rb:29:in `<main>'

Reverting d73b91986d7c fixes it.