Fix seed stub files

Fixes seed stub files so they return a Promise:
https://github.com/tgriesser/knex/commit/f8d01e99018a021737bfac8c865205aec3bc6dd9#commitcomment-8090229

Updated documentation to make it clear that seed files should be idempotent:
https://github.com/tgriesser/knex/pull/391#issuecomment-57691609