Update reference to the xblock-mentoring repo for optimized dataexport

The new ref is gsehub/xblock-mentoring@94c7a4d

#### Optimize mentoring-dataexport times by using a raw SQL cursor

The bulk reading of the Answer objects to export have been converted to chunked reading of the three needed columns from a raw DB connection cursor, executed with a slightly modified version of the Django query.

Here is the corresponding xblock-mentoring PR with more details: **gsehub/xblock-mentoring#1**

*Pinging @ormsbee and @cpennington for the review.*

*Made on behalf of @antoviaque*