Builds are triggered for non-code changes

## Issue
Builds get triggered on *any* update to the merge request, not just code pushes and comments.

### Context
- **Gitlab plugin version**: 1.5.10
- **Gitlab version**: 11.4.0
- **Jenkins version**: 2.175
- **Job type**: Matrix

### Logs & Traces
I will provide logs if needed.

### Problem description
In a prior version of the plugin, MR builds would only be triggered when the MR was created, when code was updated, or when the "magic comment" was made on the MR. As of a somewhat recent update (1.5.10, I'm not sure now), *any* change to the MR will trigger a build: change the assignee, change the title, change the description, etc.

_Describe your problem in a meaningful way_:
Making changes to the MR that should not trigger a build still triggers a build.