Currently when executing `bundle exec rake test:integration` under `activejob/`
derectory, it prints helpful info like:
```
(snip)
*** rake aj:integration:async ***
(snip)
*** rake aj:integration:delayed_job ***
(snip)
 ```
but  there is no defined `:aj` scope  in `activejob/Rakefile`,
so I think output should be like:
```
(snip)
*** rake test:integration:async ***
(snip)
*** rake test:integration:delayed_job ***
(snip)
 ```

By the way `rake test:integration` doesn't work if execute it without
prepending `bundle exec` to that command. It is probably what we should
fix too.