Fix many-to-many tutorial code

[ostriandoni]

Introduction

When I try to implement code from this tutorial, I keep getting this error message:

$ knex migrate:latest

Knex:warning - migration file "20180824170607_initial_tables.js" failed
Knex:warning - migration failed with error: alter table `authors_books` add constraint `authors_books_author_id_foreign` foreign key (`author_id`) references `authors` (`id`) - ER_CANNOT_ADD_FOREIGN: Cannot add foreign key constraint

AND

$ knex migrate:rollback

Knex:warning - migration file "20180824170607_initial_tables.js" failed
Knex:warning - migration failed with error: drop table `books` - ER_ROW_IS_REFERENCED: Cannot delete or update a parent row: a foreign key constraint fails

Motivation

To make the tutorial code running as expected.

Proposed solution

• to fix ER_CANNOT_ADD_FOREIGN: Cannot add foreign key constraint:-
  Refer to this Cannot Add Foreign Key Constraint knex/knex#245 (comment)
• to fix ER_ROW_IS_REFERENCED: Cannot delete or update a parent row: a foreign key constraint fails:-
  Reorder table drop

Current PR Issues

None

Alternatives considered

None

[ricardograca]

There is still a chance that this migration will fail with clients other than SQLite or when using a connection pool with more than 1 connection, but it's still an improvement, so many thanks.