IndexOutOfBoundsException on force push on target branch

## Issue
In jenkins I activate plugin on Merge requests and on any push on the source or target branch.

If I do a force-push on target branch, a stacktrace IndexOutOfBoundsException occurs

### Context
- **Gitlab plugin version**: 1.5.3
- **Gitlab version**: 8.3.4
- **Jenkins version**: 1.6.16
- **Job type**: Pipeline, Freestyle, Matrix, etc.

### Logs & Traces
```
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
at java.util.ArrayList.rangeCheck(ArrayList.java:653)
at java.util.ArrayList.get(ArrayList.java:429)
at com.dabsquared.gitlabjenkins.trigger.handler.push.OpenMergeRequestPushHookTriggerHandler.retrieveCauseData(OpenMergeRequestPushHookTriggerHandler.java:149)
at com.dabsquared.gitlabjenkins.trigger.handler.push.OpenMergeRequestPushHookTriggerHandler.handleMergeRequest(OpenMergeRequestPushHookTriggerHandler.java:118)
at com.dabsquared.gitlabjenkins.trigger.handler.push.OpenMergeRequestPushHookTriggerHandler.handle(OpenMergeRequestPushHookTriggerHandler.java:69)
at com.dabsquared.gitlabjenkins.trigger.handler.push.OpenMergeRequestPushHookTriggerHandler.handle(OpenMergeRequestPushHookTriggerHandler.java:45)
at com.dabsquared.gitlabjenkins.trigger.handler.push.PushHookTriggerHandlerList.handle(PushHookTriggerHandlerList.java:24)
at com.dabsquared.gitlabjenkins.trigger.handler.push.PushHookTriggerHandlerList.handle(PushHookTriggerHandlerList.java:13)
at com.dabsquared.gitlabjenkins.GitLabPushTrigger.onPost(GitLabPushTrigger.java:393)
at com.dabsquared.gitlabjenkins.webhook.build.PushBuildAction$1.performOnPost(PushBuildAction.java:72)
at com.dabsquared.gitlabjenkins.webhook.build.BuildWebHookAction$TriggerNotifier.run(BuildWebHookAction.java:54)
at hudson.security.ACL.impersonate(ACL.java:198)
at com.dabsquared.gitlabjenkins.webhook.build.PushBuildAction.execute(PushBuildAction.java:69)
at com.dabsquared.gitlabjenkins.webhook.build.BuildWebHookAction.execute(BuildWebHookAction.java:31)
at com.dabsquared.gitlabjenkins.webhook.GitLabWebHook.getDynamic(GitLabWebHook.java:44)
... 71 more
```