make arcanist backwards compatible with various conduit versions

Summary: pragmatism wins the day, though I think eventually we might want something really fancy to deal with arcanist and conduit not being up to date with respect to one another

Test Plan: php -l

Reviewers: epriestley

Reviewed By: epriestley

CC: zeeg, aran, Korvin

Maniphest Tasks: T2088

Differential Revision: https://secure.phabricator.com/D3988