Just to share an awesome GitHub feature that I learned about when doing a similar integration as you; your repository also tracks all Pull Request related commits. Really helpful when you're trying to build these sorts of tools in the future. If you set refspec to the following then you'll get automated builds for any commit activity related to all Pulls. Its saved me tons of time writing middleware to handle these things.

  +refs/pull/*:refs/remotes/origin/pr/*

The major Jenkins plugins have their respective build-on-new-commits, so you can automate these builds and just treat these like feature branches.'