Improve schema_dumper handling of :primary_key

This is generating wrong schema.rb when I tried with sqlite3 :

Before :

  create_table "items", :force => true do |t|
    t.string   "name"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

After :

  create_table "items", :id => false, :force => true do |t|
    t.string   "name"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

Thanks.