Build triggered needlessly after MR states change

## Issue
Our gitlab is set up so that each merge request must be approved by 1 or 2 reviewers before it can be merged. In addition Jenkins has to successfully build the project with the proposed changes.
When a reviewer hits the "Approve" button, the "Merge Request" webhook is triggerd, causing Jenkins to build the project again, even if it was successfully built previously.

### Context
- **Gitlab plugin version**: 1.4.3
- **Gitlab version**: 9.2.10-ee
- **Jenkins version**: 2.46.2
- **Job type**: Freestyle

### Logs & Traces
### Problem description
Gitlab sends a "Merge Request" json with action "approved".
That action is apparently not evaluated, and it is also not checked whether there were actual changes committed to the feature branch.

Expected:
Hitting "Approve" in Gitlab does not trigger a new build in Jenkins.