social course format version pointing to a date in the future

This is a regression of MDL-46755 that was detected while testing MDL-50102.

it seems that, by mistake, we allowed a future date to land into master for the social course format: 2015102100

https://github.com/moodle/moodle/commit/0d67ac8ea2a87f41653cb1bce101b839cc8c2e64#diff-b25a7f1e2b97c2e7feb3c6fa40bc95afR28

And that's clearly incorrect. So this issue is about to consider if:

1) There is any way to "downgrade" the version to the correct expected one (2015050500), or, alternatively,
2) We need to live with the wrong version until October 21th (<6 months).

Ciao