CYB-372 Comment activity not showing up in CSV export

* there was a typo in the SQL, so that type of data was not selected
  (It is 'GalleryComment', not 'Comment')

https://jira.ets.berkeley.edu/jira/browse/CYB-372
Merge pull request #227 from coyote/correct_activity_type_typo

CYB-372 Comment activity not showing up in CSV export