Add version migration support to rails 4

When switching to rails 5, we added migration version to all migration
classes. This patch makes it possible to run versioned migrations
also with rails 4

config/application.rb

@@ -26,6 +26,9 @@ module Gitlab
     # setting disabled
     require_dependency Rails.root.join('lib/mysql_zero_date')
 
+    # This can be removed when we drop support for rails 4
+    require_dependency Rails.root.join('lib/rails4_migration_version')
+
     # Settings in config/environments/* take precedence over those specified here.
     # Application configuration should go into files in config/initializers
     # -- all .rb files in that directory are automatically loaded.

lib/rails4_migration_version.rb

+# rubocop:disable Naming/FileName
+# frozen_string_literal: true
+
+# When switching to rails 5, we added migration version to all migration
+# classes. This patch makes it possible to run versioned migrations
+# also with rails 4
+
+unless Gitlab.rails5?
+  module ActiveRecord
+    class Migration
+      def self.[](version)
+        Migration
+      end
+    end
+  end
+end