Fix weird issue where INF is not respected in PHP 5.5

Summary:
After upgrading to PHP 5.5, the conduit list was not fully visible because
INF was being treated as "0" for some reason. Fixed by making it a PHP_MAX_INT

Test Plan: Checked on PHP 5.5

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7530