Fix an undefined variable

Summary:
The `$timeline` variable is undefined. I was seeing the following error in the logs:

```
EXCEPTION: (RuntimeException) Undefined variable: timeline at [<phutil>/src/error/PhutilErrorHandler.php:210]
   #0 PhutilErrorHandler::handleError(integer, string, string, integer, array) called at [<phabricator>/src/applications/harbormaster/controller/HarbormasterStepEditController.php:205]
   #1 HarbormasterStepEditController::processRequest() called at [<phabricator>/src/aphront/AphrontController.php:33]
   #2 AphrontController::handleRequest(AphrontRequest) called at [<phabricator>/webroot/index.php:103]
```

Test Plan: Created a build step without a fatal error.

Reviewers: btrahan, hach-que, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D10941