Adding support to utf8mb4 on mysql2 for schema_migrations table

Given the following database.yml:
```yaml
development: 
  adapter: mysql2
  encoding: utf8mb4
  collation: utf8mb4_unicode_ci
```
This error is thrown when I try to create a database:
```shell
ActiveRecord::StatementInvalid: Mysql2::Error: Specified key was too long; max key length is 767 bytes: CREATE UNIQUE INDEX `unique_schema_migrations` ON `schema_migrations` (`version`)
```
That's because of a limitation created by utf8mb4: indexes can be at most 191 chars and schema_migrations is a varchar(255).