Remove migrations warning

DEPRECATION WARNING: Directly inheriting from ActiveRecord::Migration
is deprecated. Please specify the Rails release the migration was
written for:

class MigrationClass < ActiveRecord::Migration[4.2]

(called from require at bin/rails:4)

db/migrate/20180718115545_create_i18n_content_translations.rb

@@ -1,4 +1,4 @@
-class CreateI18nContentTranslations < ActiveRecord::Migration
+class CreateI18nContentTranslations < ActiveRecord::Migration[4.2]
   def change
     create_table :i18n_contents do |t|
       t.string :key