change rake command example

I have tried install this plug in a production, and without a development database setting, so I got an error like:

```
$ rake redmine:plugins:migrate NAME=redmine_daily_status
(in /opt/redmine)
rake aborted!
Mysql2::Error: Access denied for user 'root'@'localhost' (using password: NO)
```

So before run `rake` command, it's safe to add `RAILS_ENV=xxx` prefix and use `bundle`.