Fix bug in AR integration

[jaredbeck]

[Fixes #789]

[jaredbeck]

Reverts the part of 2029ea3 which appears to cause issue #789

I would prefer if users were required to manually include PaperTrail::Model in each of their models, but that's a whole 'nother discussion..

[jaredbeck]

on_load is explained in activesupport-4.2.6/lib/active_support/lazy_load_hooks.rb as follows:

# lazy_load_hooks allows Rails to lazily load a lot of components and thus
# making the app boot faster. Because of this feature now there is no need to
# require <tt>ActiveRecord::Base</tt> at boot time purely to apply
# configuration. Instead a hook is registered that applies configuration once
# <tt>ActiveRecord::Base</tt> is loaded. Here <tt>ActiveRecord::Base</tt> is
# used as example but this feature can be applied elsewhere too.
#
# Here is an example where +on_load+ method is called to register a hook.
#
#   initializer 'active_record.initialize_timezone' do
#     ActiveSupport.on_load(:active_record) do
#       self.time_zone_aware_attributes = true
#       self.default_timezone = :utc
#     end
#   end
#
# When the entirety of +activerecord/lib/active_record/base.rb+ has been
# evaluated then +run_load_hooks+ is invoked. The very last line of
# +activerecord/lib/active_record/base.rb+ is:
#
#   ActiveSupport.run_load_hooks(:active_record, ActiveRecord::Base)

[batter]

I would prefer if users were required to manually include PaperTrail::Model in each of their models, but that's a whole 'nother discussion..

Agreed that would be ideal, as it does seem a bit excessive including it on every ActiveRecord instance in a project where the gem is being used. Maybe we can shoot for that for PaperTrail 6?