This is the message I get when my webhook tries to send the pull_request event to my Jenkins instance:

A problem occurred while processing the request.
Please check <a href="https://issues.jenkins-ci.org/">our bug tracker</a> to see if a similar problem has already been reported.
If it is already reported, please vote and put a comment on it to let us gauge the impact of the problem.
If you think this is a new issue, please file a new issue.
When you file an issue, make sure to add the entire stack trace, along with the version of Jenkins and relevant plugins.
<a href="http://jenkins-ci.org/content/mailing-lists">The users list</a> might be also useful in understanding what has happened.</p><h2>Stack trace</h2><pre style="margin:2em; clear:both">javax.servlet.ServletException: java.lang.IllegalArgumentException: Github Webhook event of type pull_request is not supported. Only push events are current supported

I just updated the plugin.

These are the request headers:

Request URL: ###/github-webhook/
Request method: POST
Authorization: ********
content-type: application/x-www-form-urlencoded
Expect:
User-Agent: GitHub-Hookshot/2f00e0f
X-GitHub-Delivery: 31deb900-1ba2-11e5-8476-6f14de4238e3
X-GitHub-Event: pull_request

Notice I omitted the actual URL to Jenkins instance.

I noticed this issue when trying to use the GitHub Pull Request Builder Plugin and they told me this was an issue with the GitHub Plugin. Here's the link the the issue on GitHub: https://github.com/jenkinsci/ghprb-plugin/issues/121

These are the versions of my plugins:

GitHub Pull Request Builder Plugin - 1.24.2
GitHub Plugin - 1.11.3
GitHub API Plugin - 1.68