Fix needed for proper Features export - transitions as entities

Working on an Open Atrium project, we've written a module that converts transitions to entities for use with entity reference fields. However, they are not exporting to features properly because the new fields are not included in the export.

This patch adds the critical field to the export. It is not expected that this will ever be committed; please see #2174343: Make transitions into entities, for use with entity reference fields for a more robust treatment of this issue.