Attempt to support Migration and MigrationContext with Rails Version Dependency

[simonireilly]

As of Rails 5.2, the migration module has been changed.

closes #15

Instead of having paths, it now has a separate ActiveRecord::MigrationContext.

This PR is the initial work to build for rails 5.2.3

[simonireilly]

This module is moved into its own file from migrator.rb

[simonireilly]

The new migration context, as required by Rails 5.2.

This will need some work as it must act as the new module does in rails; for now it is just a placeholder

[simonireilly]

@ryantownsend @paulspringett @adamdullenty

I am not an ActiveRecord Savant, so would appreciate some guidance on getting this merged.

Although it has a green build for 5.2.3 I suspect there is some missing test coverage that the new modules should have broken.

I might suggest that we use the Rails::Engine style testing for this as we have a railtie in here anyway, and we could have a dummy application that requires the new version and attempts a basic penthouse boot. WDYT?

That could be a seperate PR ahead of this which would:

1. Bring test coverage up to >95%
2. Add a dummy app testing path as per the normal way engines are tested

[simonireilly]

Suggested change

	def self.included(base)
	base.class_eval do
	# Verbose form of alias_method_chain which is now deprecated in ActiveSupport.
	#
	# This replaces the original #annouce method with #announce_with_penthouse
	# but allows calling #annouce by using #announce_without_penthouse.
	alias_method :announce_without_penthouse, :announce
	alias_method :announce, :announce_with_penthouse
	end
	end
	def announce_with_penthouse(message)
	announce_without_penthouse("#{message} - #{current_tenant}")
	end
	def announce(message)
	super("#{message} - #{current_tenant}")
	end

[simonireilly]

Suggested change

	ActiveRecord::Migration.send(:include, Penthouse::Migration)
	ActiveRecord::Migration.send(:prepend, Penthouse::Migration)