`master` is broken for more than 2 days (Mysql2::Error: Invalid default value for 'created_at')

Since https://gitlab.com/gitlab-org/gitlab-ce/commit/9c11894a656603c59c03d8f35fbe8cdf7a1ad0c4 (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13433), master is consistently broken for 4 MySQL jobs!

@smcgivern Can we revert this MR if we don't have a fix in the next hours?

@yorickpeterse What do you think about adding the following to the the Database checklist?:

Branch name includes mysql name to run the test suite against MySQL, to ensure we don't break MySQL support.