Fix stdout value to 'pipe'

Result of using a `RegExp` in the `options.ready`,
```
$ grunt run --stack
Running "run:run" (run) task
Warning: Cannot call method 'on' of null Use --force to continue.
TypeError: Cannot call method 'on' of null
    at Object.<anonymous> (./node_modules/grunt-run/tasks/run.js:120:21)
    at Object.<anonymous> (./node_modules/grunt/lib/grunt/task.js:264:15)
...
```