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.
fixes broken test
ensure we build on the right commit
Merge remote-tracking branch 'upstream/pr/94'

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

adds build status and more description around the plugin
Merge pull request #4 from janinko/master

Latest from master