Don't require the "gd" extension be installed in order to run unit tests
Ref T10405. If gd is not installed, a number of unit tests currently fail because they generate synthetic users and try to composite profile pictures for them.
Instead, just fall back to a static default if gd is not available.
Test Plan: Faked this pathway, created a new user, saw a default profile picture.
Reviewers: amckinley, lpriestley, avivey
Reviewed By: avivey
Maniphest Tasks: T10405
Differential Revision: https://secure.phabricator.com/D18812