Update GhprbTrigger.java

Just spent some time tracking down the cause of this issue which started appearing after someone had disabled the GitHub plugin. The wording change will make the cause of this issue clearer to the user to allow for a quicker resolution.
Merge remote-tracking branch 'upstream/pr/94'

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