Only include active users for Ares CourseUsers

The Ares CourseUsers file does not distinguish between inactive and
active users. Only send active users.

This change was originally implemented in commit
8fe7587c8d6e8490ce5058e9b0c51ce1f5eed15a, however the predicate was not
passed to ares_feed as a parameter, so it was never actually used.
Fixes #18.