Fix deletion of empty directories

Add routing guide doc on root with namespaces. [ci skip]

Fix touching an invalid parent record for belongs_to

If the parent of a `belongs_to` record fails to be saved due to
validation errors, `touch` will be called on a new record, which causes
an exception (see https://github.com/rails/rails/pull/9320).

Example:

    class Owner < ActiveRecord::Base
      validates_presence_of :name
    end

    class Pet < ActiveRecord::Base
      belongs_to :owner, touch: true
    end

    pet = Pet.new(owner: Owner.new)

    # Before, this line would raise ActiveRecord::ActiveRecordError
    # "can not touch on a new record object"
    pet.save

I read #9419 and realized I didn't know you could have namespaced
roots, checked around, and a few friends didn't either.  Figured
this makes it guide worthy.

[ci skip]

Journey::Path::Pattern#new raise more meaningful exception message.

Use less hyperbole in the language re: allowed parameters

Add tests for #9441

This is a small thing, motivated by https://github.com/rails/rails/issues/9419

Expand order(:symbol) to "table".symbol to prevent broken queries on PG.

JRuby does not support racc, include it only in ruby platform

Change default root route helper

small typo on upgrade guide

small typo fix

1. When comparing the directory to delete against the top level
   cache_path, use File.realpath to make sure we aren't comparing two
   unequal strings that point to the same path. This occurs, for
   example, when cache_path has a trailing slash, which it does in the
   default Rails configuration. Since the input to
   delete_empty_directories never has a trailing slash, the comparison
   will never be true and the top level cache directory (and above) may
   be deleted. However…

2. File.delete raises EPERM when trying to delete a directory, so no
   directories have ever been deleted. Changing the code to Dir.delete
   fixes that.

It turns out this file is required in active_record.rb.

Remove sprockets-rails from the Gemfile generator