Fixed "Notice: Undefined variable: connection in plugins/migrations/libs/model/cake_migration.php on line 296".

57eea8f501011ad70f84f11205f781eca3697e42 has a bug.
When I run migration, an notice occurred.

```
PHP Notice:  Undefined variable: connection in /plugins/migrations/libs/model/cake_migration.php on line 296
```

It's because the variable `$connection* is not defined, cannot work to use a different db specified in the migration file for `alter table`.
I defined *$connection`.