Use phutil_json_decode instead of json_decode
Summary: Generally, phutil_json_decode should be preferred over json_decode.
Test Plan: Eyeballed.
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: aurelijus, Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D12678