Fixed regression introduced in PR rails#39.

- Also added test so it won't happen again.
- Added bundler config to fix insecure git protocol warnings.
- Removed a couple of unneeded deps.

Resolves rails#45
Resolves rails#49

.bundle/config

@@ -0,0 +1,2 @@
+---
+BUNDLE_GITHUB__HTTPS: "true"

.gitignore

@@ -1,6 +1,7 @@
 *.gem
 *.rbc
-.bundle
+.bundle/*
+!.bundle/config
 .config
 .yardoc
 Gemfile.lock

Gemfile

@@ -3,6 +3,6 @@ source 'https://rubygems.org'
 # Specify your gem's dependencies in active_record-observers.gemspec
 gemspec
 gem 'rails', github: 'rails/rails', branch: '5-0-stable'
-gem 'activeresource', github: 'rails/activeresource'
+gem 'activeresource', github: 'rails/activeresource', branch: 'master'
 
 gem 'mocha', require: false

lib/rails/observers/active_model/observing.rb

@@ -1,9 +1,8 @@
+require 'set'
 require 'singleton'
 require 'rails/observers/active_model/observer_array'
-require 'active_support/core_ext/module/aliasing'
 require 'active_support/core_ext/module/remove_method'
 require 'active_support/core_ext/string/inflections'
-require 'active_support/core_ext/enumerable'
 require 'active_support/core_ext/object/try'
 require 'active_support/descendants_tracker'
 
@@ -13,7 +12,11 @@ module Observing
     extend ActiveSupport::Concern
 
     included do
-      extend ActiveSupport::DescendantsTracker
+      extend ::ActiveSupport::DescendantsTracker
+      def self.inherited(subclass)
+        super
+        subclass.observer_orm.instantiate_observers
+      end
     end
 
     module ClassMethods
@@ -74,7 +77,7 @@ def observers
       #
       #   Foo.observer_instances # => [#<FooObserver:0x007fc212c40820>]
       def observer_instances
-        @observer_instances ||= []
+        @observer_instances ||= Set.new
       end
 
       # Instantiate the global observers.
@@ -176,6 +179,17 @@ def count_observers
         observers_count
       end
 
+      # Inheritable, read-only, lazily-determined, memoized class variable reader
+      # referring to the Active.*::Base class that included ::ActiveModel::Observing
+      def observer_orm #:nodoc:#
+        @@observer_orm ||= begin
+          self_and_parents = self.parents.unshift(self)
+          self_and_parents.detect do |klass|
+            klass.included_modules.include?(::ActiveModel::Observing)
+          end
+        end
+      end
+
       protected
         def instantiate_observer(observer) #:nodoc:
           # string/symbol