Feature: Cancel pending builds for this pull request

The intent is to not start builds that are not the tip of the pull request, useful for builds which take a long time to run.
There is a findPreviousBuildForPullId method in GhprbTrigger: https://github.com/jenkinsci/ghprb-plugin/blob/master/src/main/java/org/jenkinsci/plugins/ghprb/GhprbTrigger.java#L204