title: Jenkins won't trigger build after merging a pull request in GitHub
body: <p>So far triggering a build in Jenkins work's fine for creating a new pull request, pushing changes and leaving a comment like <code>test this please</code> however, when I hit green "Merge pull request" button in GitHub, I don't see any jobs running in Jenkins.</p>
  
  <p><strong>MY JOB SETTINGS</strong></p>
  
  <p>Refspec: <code>+refs/pull/*:refs/remotes/origin/pr/*</code></p>
  
  <p>Branch Specifier: <code>origin/master</code> or <code>${sha1}</code></p>
  
  <p>Took [GitHub pull request builder plugin documentation][1] as reference.</p>
  
  <p><strong>JENKINS LOGS</strong></p>
  
  <pre><code>Jan 14, 2017 10:50:17 PM org.jenkinsci.plugins.ghprb.GhprbRootAction handleAction
  INFO: Checking PR #3 for Username/project
  Jan 14, 2017 10:50:17 PM org.jenkinsci.plugins.ghprb.GhprbTrigger handlePR
  INFO: Checking PR #3 for job Nation
  Jan 14, 2017 10:50:17 PM org.jenkinsci.plugins.github.webhook.subscriber.DefaultPushGHEventSubscriber onEvent
  INFO: Received PushEvent for https://github.com/Username/project
  Jan 14, 2017 10:50:17 PM org.jenkinsci.plugins.ghprb.GhprbRootAction handleAction
  WARNING: Request not known for event: push
  
  Jan 14, 2017 10:50:28 PM org.jenkinsci.plugins.ghprb.GhprbRootAction handleAction
  WARNING: Request not known for event: push
  Jan 14, 2017 10:50:28 PM org.jenkinsci.plugins.github.webhook.subscriber.DefaultPushGHEventSubscriber onEvent
  INFO: Received PushEvent for https://github.com/Username/project
  </code></pre>
  
  <p><strong>GITHUB WEBHOOK LOGS</strong></p>
  
  <p><strong>Log 1)</strong>
  Request
  <code>X-GitHub-Event: pull_request</code>
  Payload
  <code>.... "action": "closed" ...</code></p>
  
  <p><strong>Log 2)</strong>
  Request
  <code>X-GitHub-Event: push</code>
  Payload
  <code>.... "ref": "refs/heads/develop" ...</code></p>
  
  <p><strong>Log 3)</strong>
  Request
  <code>X-GitHub-Event: push</code>
  Payload
  <code>.... "ref": "refs/heads/feature/update-gitignore" ...</code></p>
  
  <hr>
  
  <p><strong>UPDATE</strong></p>
  
  <p>I have <code>Pull request</code>, <code>Issue comment</code> and <code>Push</code> events are enabled in GitHub Webhook Setting page.</p>