Fix rake tasks: db:create, db:drop and db:migrate

- [x] fix `db:create`, `db:drop` and `db:migrate` tasks
- [x] `ActiveRecord::SchemaMigration.columns` shouldn't switch to shard. It feels like ARS should support scenario where a model lives in not sharded and sharded databases, but I'm not sure if there're many use cases other than `schema_migrations`
- [x] create and drop slave databases. Why migrations [are not executed](https://github.com/zendesk/active_record_shards/blob/lukkry/fix_rake_tasks/lib/active_record_shards/migration.rb#L6-L11) on slaves? 

Keeping master compatible with rails 3.2.x and 4.0.x makes code messy ;(