class AddTagsToArticles < ActiveRecord::Migration[4.2]
  def change
  	add_column :articles, :tags, :string, array: true, default: []
  end
end

class AddTagsToArticles < ActiveRecord::Migration[4.2]
  def change
  	add_column :articles, :tags, :string, default: [], array: true
  end
end

Move array: true to the end of the migration
Move array: true to the end of the migration
