List columns within the method (#11377)

To avoid the exception:

NoMethodError: undefined method `perform' for nil:NilClass
.../vendor/bundle/ruby/2.6.0/gems/strong_migrations-0.4.1/lib/strong_migrations/migration.rb:14:in `method_missing'
.../vendor/bundle/ruby/2.6.0/gems/activerecord-5.2.3/lib/active_record/migration.rb:604:in `method_missing'
.../db/migrate/20170918125918_ids_to_bigints.rb:69:in `<class:IdsToBigints>'
.../db/migrate/20170918125918_ids_to_bigints.rb:3:in `<top (required)>'

db/migrate/20170918125918_ids_to_bigints.rb

@@ -5,7 +5,8 @@ class IdsToBigints < ActiveRecord::Migration[5.1]
 
   disable_ddl_transaction!
 
-  INCLUDED_COLUMNS = [
+  def migrate_columns(to_type)
+    included_columns = [
       [:account_domain_blocks, :account_id],
       [:account_domain_blocks, :id],
       [:accounts, :id],
@@ -66,9 +67,8 @@ class IdsToBigints < ActiveRecord::Migration[5.1]
       [:web_settings, :id],
       [:web_settings, :user_id],
     ]
-  INCLUDED_COLUMNS << [:deprecated_preview_cards, :id] if table_exists?(:deprecated_preview_cards)
+    included_columns << [:deprecated_preview_cards, :id] if table_exists?(:deprecated_preview_cards)
 
-  def migrate_columns(to_type)
     # Print out a warning that this will probably take a while.
     say ''
     say 'WARNING: This migration may take a *long* time for large instances'
@@ -86,7 +86,7 @@ class IdsToBigints < ActiveRecord::Migration[5.1]
       sleep 1
     end
 
-    tables = INCLUDED_COLUMNS.map(&:first).uniq
+    tables = included_columns.map(&:first).uniq
     table_sizes = {}
 
     # Sort tables by their size
@@ -94,7 +94,7 @@ class IdsToBigints < ActiveRecord::Migration[5.1]
       table_sizes[table] = estimate_rows_in_table(table)
     end
 
-    ordered_columns = INCLUDED_COLUMNS.sort_by do |col_parts|
+    ordered_columns = included_columns.sort_by do |col_parts|
       [-table_sizes[col_parts.first], col_parts.last]
     end