MR source branch removal broken by GitLab 9.2

Since gitlab-ce MR 10319, whose changes were first included in GitLab 9.2, there is no interface for the user to set the force_remove_source_branch value. Instead the source branch removal is determined by a check box next to the merge button widget. This means that our host::MergeRequest structure populated by the ghostflow-gitlab crate will never have remove_source_branch set to true.