Labeling an open MR triggers a build

## Issue
When making minor changes to an open merge request that should not affect the result of the Jenkins run, a rebuild is triggered still.
This includes adding/removing a label, setting/removing a milestone, as well as editing the description (or title) of the MR.

**Note:** This issue is closely related to issue #696, which was closed as a duplicate to #633, which did not fix the issues described here (or in #696).

### Context
- **Gitlab plugin version**: 1.5.5
- **Gitlab version**: 10.4.6 EE
- **Jenkins version**: 2.138.2
- **Job type**: Freestyle

### Logs & Traces

Here's a log extract from Jenkins.
The relevant part is the `changes` section. In this case, I only changed the Milestone assignment for the MR:
```
WebHook called with url: /project/navi-builder

Dec 17, 2018 8:37:36 AM FINE com.dabsquared.gitlabjenkins.webhook.build.MergeRequestBuildAction

MergeRequest: {
"object_kind" : "merge_request",
"user" : {
"name" : "Frank Aurich",
"username" : "f.aurich",
"avatar_url" : ***FILTERED***
},
"project" : {
"id" : 1242,
"name" : ***FILTERED***,
"description" : "",
"web_url" : ***FILTERED***,
"avatar_url" : null,
"git_ssh_url" : ***FILTERED***,
"git_http_url" : ***FILTERED***,
"namespace" : ***FILTERED***,
"visibility_level" : 10,
"path_with_namespace" : ***FILTERED***,
"default_branch" : "mw_develop",
"ci_config_path" : null,
"homepage" : ***FILTERED***,
"url" : ***FILTERED***,
"ssh_url" : ***FILTERED***,
"http_url" : ***FILTERED***
},
"object_attributes" : {
"assignee_id" : 316,
"author_id" : 316,
"created_at" : "2018-12-14 17:18:58 +0100",
"description" : ***FILTERED***,
"head_pipeline_id" : 61912,
"id" : 26510,
"iid" : 63,
"last_edited_at" : "2018-12-14 17:19:30 +0100",
"last_edited_by_id" : 316,
"merge_commit_sha" : null,
"merge_error" : null,
"merge_params" : {
"force_remove_source_branch" : "1"
},
"merge_status" : "can_be_merged",
"merge_user_id" : null,
"merge_when_pipeline_succeeds" : false,
"milestone_id" : 4676,
"source_branch" : "bugfix/DEFECT-34585",
"source_project_id" : 1242,
"state" : "opened",
"target_branch" : "mw_develop",
"target_project_id" : 1242,
"time_estimate" : 0,
"title" : ***FILTERED***,
"updated_at" : "2018-12-17 08:37:35 +0100",
"updated_by_id" : 83,
"url" : ***FILTERED***,
"source" : {
"id" : 1242,
"name" : ***FILTERED***,
"description" : "",
"web_url" : ***FILTERED***,
"avatar_url" : null,
"git_ssh_url" : ***FILTERED***,
"git_http_url" : ***FILTERED***,
"namespace" : ***FILTERED***,
"visibility_level" : 10,
"path_with_namespace" : ***FILTERED***,
"default_branch" : "mw_develop",
"ci_config_path" : null,
"homepage" : ***FILTERED***,
"url" : ***FILTERED***,
"ssh_url" : ***FILTERED***,
"http_url" : ***FILTERED***
},
"target" : {
"id" : 1242,
"name" : ***FILTERED***,
"description" : "",
"web_url" : ***FILTERED***,
"avatar_url" : null,
"git_ssh_url" : ***FILTERED***,
"git_http_url" : ***FILTERED***,
"namespace" : ***FILTERED***,
"visibility_level" : 10,
"path_with_namespace" : ***FILTERED***,
"default_branch" : "mw_develop",
"ci_config_path" : null,
"homepage" : ***FILTERED***,
"url" : ***FILTERED***,
"ssh_url" : ***FILTERED***,
"http_url" : ***FILTERED***
},
"last_commit" : {
"id" : "1e2c74baffb976fe16e8cfe2490826609009a1a8",
"message" : ***FILTERED***,
"timestamp" : "2018-12-14T17:10:24+01:00",
"url" : ***FILTERED***,
"author" : {
"name" : ***FILTERED***,
"email" : ***FILTERED***
}
},
"work_in_progress" : false,
"total_time_spent" : 0,
"human_total_time_spent" : null,
"human_time_estimate" : null,
"action" : "update"
},
"labels" : [ ],
"changes" : {
"milestone_id" : {
"previous" : null,
"current" : 4676
},
"updated_at" : {
"previous" : "2018-12-17 08:35:19 +0100",
"current" : "2018-12-17 08:37:35 +0100"
},
"updated_by_id" : {
"previous" : 316,
"current" : 83
}
},
"repository" : {
"name" : ***FILTERED***,
"url" : ***FILTERED***,
"description" : "",
"homepage" : ***FILTERED***
},
"assignee" : {
"name" : ***FILTERED***,
"username" : ***FILTERED***,
"avatar_url" : ***FILTERED***
}
}

Dec 17, 2018 8:37:36 AM INFO com.dabsquared.gitlabjenkins.trigger.handler.AbstractWebHookTriggerHandler handle

navi-builder triggered for merge request.
```
Here is `changes` section after a new label has been added to an MR:
```
[...]
"changes" : {
"labels" : {
"previous" : [ ],
"current" : [ {
"id" : 3348,
"title" : "X137",
"color" : "#44AD8E",
"project_id" : null,
"created_at" : "2018-12-17 08:40:27 +0100",
"updated_at" : "2018-12-17 08:40:27 +0100",
"template" : false,
"description" : "Relevant SW change for SW137",
"type" : "GroupLabel",
"group_id" : 43
} ]
}
},
[...]
```

### Problem description

In Gitlab, a webhook is configured to be triggered on `Merge Request` and `Comment`.
**Expected:**
Jenkins starts a new build job when new commits to the MR branch occur, or when someone triggers the Jenkins manually via a special comment in Gitlab.
**Actual:**
Jenkins triggers a job when changes are made to the MR that are not relevant to the Jenkins build:
label changes, milestone changes, changes to the description of the MR, changes to the number of approvers (Gitlab EE).