Switch from merge to merge_when_pipeline_succeeds in merge_request_api.sh

Description of bug

Merging automatically from the pipeline didn't work, because we ask for the pipelines to succeed before merging, so the PUT request returns "method not allowed"

See #186 (closed) and !2001 (merged)

Changes introduced

Instead of asking for the pipeline to merge, we convert the request into a merge_when_pipeline_succeeds

Tests

Not available - at least I know that the PUT request works because !2001 (merged) was merged automatically - also I tried this in this MR and it seems to be working:

Issues resolved

#186 (closed) - will close this after it has been tested again

@zbhatti have a look at what happens next time you make such a MR. If it is not merged automatically, it means that the is still some work to be done.