Log warning about courses added during cron is falsely reporting all courses

[ssciolla]

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Expected behavior (A description of what you expected to happen) :

The log warning near the end of the run of cron.py should warn users only about courses that were added to the system during the cron run.

Describe the bug (Tell us what happens instead of the expected behavior) :

During cron runs, all of the course IDs are reported by this log message. This seems to be because of a cast of course IDs
to strings added in this commit: 141081a

[jennlove-um]

testing passes in beta