spec: remove bullet-instructure gem

Change-Id: I0881a9f5329b5c594fe55127548d4a54ba45ad60
Reviewed-on: https://gerrit.instructure.com/36964
Reviewed-by: Hannah Bottalla <hannah@instructure.com>
Product-Review: Hannah Bottalla <hannah@instructure.com>
QA-Review: Hannah Bottalla <hannah@instructure.com>
Tested-by: Hannah Bottalla <hannah@instructure.com>