Fix instantiating observers on app boot.

[kaspth]

When adding support for Rails 4.1, 4.2 and 5.0, the instantiate_observers
call that happened after boot was erroneously removed.
https://github.com/rails/rails-observers/pull/39/files#diff-9db46e46113d0c2eeb41f5d6f08bcac4L24

This covered a regression in the test suite for the rake integration and
also broke the gem for apps.

The reason the test/rake_test.rb failed was that a require to active_record/base
had been removed which rails-observers depended on.
rails/rails@9e9793b

Without that we're this circular dependency error:

kaspers-macbook-pro:observer kasperhansen$ cat Rakefile

require_relative 'config/application'

namespace :user do
  task :count => :environment do
    puts User.count
  end
end

Rails.application.load_tasks

kaspers-macbook-pro:observer kasperhansen$ bin/rake user:count
rake aborted!
Circular dependency detected while autoloading constant User
/Users/kasperhansen/Documents/code/rails-observers/lib/rails/observers/active_model/observing.rb:334:in `observed_class'
/Users/kasperhansen/Documents/code/rails-observers/lib/rails/observers/active_model/observing.rb:325:in `observed_classes'
/Users/kasperhansen/Documents/code/rails-observers/lib/rails/observers/active_model/observing.rb:345:in `observed_classes'
/Users/kasperhansen/Documents/code/rails-observers/lib/rails/observers/activerecord/observer.rb:100:in `observed_classes'
/Users/kasperhansen/Documents/code/rails-observers/lib/rails/observers/active_model/observing.rb:341:in `initialize'
/Users/kasperhansen/Documents/code/rails-observers/lib/rails/observers/active_model/observing.rb:186:in `instantiate_observer'
/Users/kasperhansen/Documents/code/rails-observers/lib/rails/observers/active_model/observing.rb:108:in `block in instantiate_observers'
/Users/kasperhansen/Documents/code/rails-observers/lib/rails/observers/active_model/observing.rb:108:in `each'
/Users/kasperhansen/Documents/code/rails-observers/lib/rails/observers/active_model/observing.rb:108:in `instantiate_observers'
/Users/kasperhansen/Documents/code/rails-observers/lib/rails/observers/railtie.rb:42:in `block (2 levels) in <class:Railtie>'
/Users/kasperhansen/Documents/code/observer/app/models/application_record.rb:1:in `<top (required)>'
/Users/kasperhansen/Documents/code/observer/app/models/user.rb:1:in `<top (required)>'
/Users/kasperhansen/Documents/code/observer/Rakefile:8:in `block (2 levels) in <top (required)>'
Tasks: TOP => user:count
(See full trace by running task with --trace)

That's because User loads ActiveRecord::Base which then instantiates
the observers in its on_load hook, finally calling observed_class
which constantizes the very User constant we're loading!

One fix is to add a require to active_record/base manually here, because
we depend on the load hooks having been fired before model constants are
loaded.

(I didn't look into reorganizing the code such that observed_class isn't
needed on observer instantiation.)

This has the added benefit of not breaking the gem should Rails remove
the require's in console or runner blocks as well.

I can reliably reproduce the above circular dependency error by commenting
out the added require "active_record/base". And when it's in the rake
task works:

kaspers-macbook-pro:observer kasperhansen$ bin/rake user:count
1

[kaspth]

@rafaelfranca you worked on rails/rails@9e9793b, but don't the console and runner blocks have the same problem of triggering set_configs too soon?

https://github.com/rails/rails/blob/83f39a3bcf3f50d12d41bff4a110c57e2e4be605/activerecord/lib/active_record/railtie.rb#L52-L60