Also define RACK_ENV for migrate task

When using ActiveRecord outside of Rails with for example https://github.com/janko-m/sinatra-activerecord you need to set `RACK_ENV` since Sinatra does not look at `RACK_ENV`.

This quick fix puts both `RAILS_ENV` and `RACK_ENV` in the command.

Another way might be to define which variable to use (`RAILS_ENV` or `RACK_ENV`) throughout the whole script.