[assets][scheduler] Minor bugfixes. See changelog below

CHANGELOG
=========
* Remove rivets binding from the parents input field in
  `job_detail_view.hbs`, the `JobDetailView` template since data updates
  are handled by `select2`.
* Update `DependencyBasedJob#parents` representation to be `Set[String]`,
  rather than a `List[String]`
* Update tests to account for changes to `DependencyBasedJob#parents`
Merge pull request #44 from airbnb/minor-bugfixes

[assets][scheduler] Minor bugfixes. See changelog below