update issue page

db/migrate/20151216142142_acts_as_taggable_on_migration.acts_as_taggable_on_engine.rb

-# This migration comes from acts_as_taggable_on_engine (originally 1)
-class ActsAsTaggableOnMigration < ActiveRecord::Migration
-  def self.up
-    create_table :tags do |t|
-      t.string :name
-    end
-
-    create_table :taggings do |t|
-      t.references :tag
-
-      # You should make sure that the column created is
-      # long enough to store the required class names.
-      t.references :taggable, polymorphic: true
-      t.references :tagger, polymorphic: true
-
-      # Limit is created to prevent MySQL error on index
-      # length for MyISAM table type: http://bit.ly/vgW2Ql
-      t.string :context, limit: 128
-
-      t.datetime :created_at
-    end
-
-    add_index :taggings, :tag_id
-    add_index :taggings, [:taggable_id, :taggable_type, :context]
-  end
-
-  def self.down
-    drop_table :taggings
-    drop_table :tags
-  end
-end