Fix "Send an email to" Herald rule for Differential

Summary: Herald returns a map of `phid => true`. This is unintuitive and should probably be cleaned up eventually.

Test Plan: With a "Send an email to" rule, updated a revision and saw no error in error log.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Differential Revision: https://secure.phabricator.com/D8527