Return the build from applyPlan instead of the plan

Summary: Nothing inside Phabricator uses the return value of this method, but returning the actual build instance is far more useful (for kicking off builds in an application and storing the build PHID against another object).

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9494