Ensure Maniphest CC PHID list is always a list in maniphest.info

Summary: See T626. Use array_values() to discard keys, for consistency and so
this will always encode as a list (JSON array) over the wire.

Test Plan: Added and removed CCs from a task while calling maniphest.info on it;
CCs worked and I always received a list.

Reviewers: btrahan, jungejason, nh, tuomaspelkonen, aran

Reviewed By: aran

CC: skrul, aran, btrahan, epriestley

Differential Revision: 1118