Add precision: 6 to books.{created,updated}_at test table

Some tests will require precision.

activerecord/test/schema/schema.rb

@@ -119,8 +119,13 @@ ActiveRecord::Schema.define do
     t.index :isbn, where: "published_on IS NOT NULL", unique: true
     t.index "(lower(external_id))", unique: true if supports_expression_index?
 
-    t.datetime :created_at
-    t.datetime :updated_at
+    if supports_datetime_with_precision?
+      t.datetime :created_at, precision: 6
+      t.datetime :updated_at, precision: 6
+    else
+      t.datetime :created_at
+      t.datetime :updated_at
+    end
     t.date :updated_on
   end