Fix AR::Migrator to create schema_migrations table for MySQL "utf8mb4" encoding

conditions:

* Mysql2Adapter
* encoding: "utf8mb4"

steps to reproduce:

```sh
rake db:create
rake db:migrate
```

>    Mysql2::Error: Specified key was too long; max key length is 767 bytes: CREATE UNIQUE INDEX `unique_schema_migrations`  ON `schema_migrations` (`version`)

This commit https://github.com/rails/rails/commit/8744632fb5649cf26cdcd1518a3554ece95a401b fixed `db:schema:load`, but didn't apply to `db:migrate`.

refs #9855