Add migration versions (#3574)

Since Rails 5.1 missing migration version results in following error:

```
StandardError: Directly inheriting from ActiveRecord::Migration is not supported. Please specify the Rails release the migration was written for:
```

This PR fixes all migration files.

db/migrate/20161006213403_rails_settings_migration.rb

 MIGRATION_BASE_CLASS = if ActiveRecord::VERSION::MAJOR >= 5
   ActiveRecord::Migration[5.0]
 else
-  ActiveRecord::Migration
+  ActiveRecord::Migration[4.2]
 end
 
 class RailsSettingsMigration < MIGRATION_BASE_CLASS

db/migrate/20170112154826_migrate_settings.rb

-class MigrateSettings < ActiveRecord::Migration
+class MigrateSettings < ActiveRecord::Migration[4.2]
   def up
     remove_index :settings, [:target_type, :target_id, :var]
     rename_column :settings, :target_id, :thing_id

db/migrate/20170330164118_add_attachment_data_to_imports.rb

-class AddAttachmentDataToImports < ActiveRecord::Migration
+class AddAttachmentDataToImports < ActiveRecord::Migration[4.2]
   def self.up
     change_table :imports do |t|
       t.attachment :data