Use versioned migrations when possible

Rails 5 deprecates inheriting directly from `ActiveRecord::Migration` in
favor of inheriting from `ActiveRecord::Migration[5.0]` where `5.0` is
the `major.minor` version of Rails that the migration was originally
written to support.

h/t to b0ce189c69.

lib/generators/active_record/devise_generator.rb

@@ -11,9 +11,9 @@ module ActiveRecord
 
       def copy_devise_migration
         if (behavior == :invoke && model_exists?) || (behavior == :revoke && migration_exists?(table_name))
-          migration_template "migration_existing.rb", "db/migrate/add_devise_to_#{table_name}.rb"
+          migration_template "migration_existing.rb", "db/migrate/add_devise_to_#{table_name}.rb", migration_version: migration_version
         else
-          migration_template "migration.rb", "db/migrate/devise_create_#{table_name}.rb"
+          migration_template "migration.rb", "db/migrate/devise_create_#{table_name}.rb", migration_version: migration_version
         end
       end
 
@@ -86,6 +86,12 @@ RUBY
         config = ActiveRecord::Base.configurations[Rails.env]
         config && config['adapter'] == 'postgresql'
       end
+
+     def migration_version
+       if rails5?
+         "[#{Rails::VERSION::MAJOR}.#{Rails::VERSION::MINOR}]"
+       end
+     end
     end
   end
 end

lib/generators/active_record/templates/migration.rb

@@ -1,4 +1,4 @@
-class DeviseCreate<%= table_name.camelize %> < ActiveRecord::Migration
+class DeviseCreate<%= table_name.camelize %> < ActiveRecord::Migration<%= migration_version %>
   def change
     create_table :<%= table_name %> do |t|
 <%= migration_data -%>

lib/generators/active_record/templates/migration_existing.rb

@@ -1,4 +1,4 @@
-class AddDeviseTo<%= table_name.camelize %> < ActiveRecord::Migration
+class AddDeviseTo<%= table_name.camelize %> < ActiveRecord::Migration<%= migration_version %>
   def self.up
     change_table :<%= table_name %> do |t|
 <%= migration_data -%>