Fix to help with the changelog for a PR build.

This adds a BuildData as an Action for a build that represents the last
build for this particular PR.  This ensures that GitSCM uses the
appropriate revision when determining the changelog.
Merge remote-tracking branch 'valdisrigdon/changelog'
remove unused imports

Conflicts:
	src/main/java/org/jenkinsci/plugins/ghprb/GhprbBuilds.java
	src/main/java/org/jenkinsci/plugins/ghprb/GhprbTrigger.java