add merge_commit_sha to Cause

## Issue

### Context
- **Gitlab plugin version**: 1.4.6
- **Gitlab version**: 9.3.10
- **Jenkins version**: 1.6.x
- **Job type**: Pipeline

### Logs & Traces

None

### Problem description

When merging an MR, GitLab sends the `merge_commit_sha` as part of the WebHook:

```
{
"object_kind": "merge_request",
…
"object_attributes": {
…
"merge_commit_sha": "eaafb2a11e5cbd08ba46dee9540275136c86e590",
},
…
}
```

It would be awesome if that would be available as part of the injected environment variables.