Found that the id from the ObjectAttribute dit not point to the id of the merge request. In my cases the iid has the number of the merge request.

This commit adds the iid to the ObjectAttribute and changes the GitLabMergerCause description to us this instead of the id.
Merge pull request #34 from kasper-f/correct_gitlab_merge_number

Found that the id from the ObjectAttribute dit not point to the id of th...