set ScribdMimeType shard category for rails 3

Change-Id: I203fb91aa9f7e4a158c06154164ce07ad69b41aa
Reviewed-on: https://gerrit.instructure.com/28972
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>