Utilize phutil_json_decode

Summary: Use phutil_json_decode instead of json_decode to throw a more tailored exception.

Test Plan: Broke xhpast to return invalid JSON... saw a PhutilProxyException thrown.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D12284