Fix setup from getRequiredClasses buggyboo

Summary:
D2470 added Package mailhandler, which was configured incorrectly in the getRequiredClasses function. this makes it like the other mail handlers

Reported at https://github.com/facebook/phabricator/issues/112

Test Plan: setup mode no longer fails

Reviewers: epriestley, jungejason, royklopper

Reviewed By: royklopper

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2476