Augmented DifferentailDependsOnFieldSpecification to expose its data over conduit

Summary: Implmented conduit methods from base class in DifferentialDependsOnFieldSpecification

Test Plan: Created two test diffs, made one of them dependent on another. Then checked the dependency over conduit interface for differential.query. Dependency is shown but in terms of PHIDs

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5238