trigger-jenkins-build: DRY up event handlers
trigger-jenkins-build: DRY up event handlers

PR-URL: https://github.com/nodejs/github-bot/pull/191
Reviewed-By: Phillip Johnsen <470e1382ee18f73c6e6136d311417869a87d7cf6@gmail.com>
phillipj/github-bot,nodejs/github-bot