just add redshift adapter(blank file)

## before

```ruby
[5] pry(main)> HogeOnRedshift
LoadError: cannot load such file -- /Users/xxx/.rbenv/versions/xxx/lib/ruby/gems/xxx/gems/activerecord-import-0.2.9/lib/activerecord-import/active_record/adapters/redshift_adapter
```

## after
```ruby
[1] pry(main)> HogeOnRedshift
=> SearchSession(id: integer ...)
```

## related issue

https://github.com/zdennis/activerecord-import/issues/89