Use assert_instances_of(, 'array')

Summary: To have at least one real callsite.

Test Plan:
  $message = new ArcanistLintMessage();
  $message->setOtherLocations(array());
  $message->setOtherLocations(array(array()));
  $message->setOtherLocations(array(1));

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4419