remove the pattern plain: from gerrit trigger!

[khozema-nullwala]

Hi, In the DevOps Academy session while running through Module - 02 lab activity the build job does not gets trigger.

Removing the plain: from the gerrit trigger pattern of Reference Application Build job then after saving changes to this job it gets executed on code push done to master branch.

[RobertNorthard]

Thanks @khozema-nullwala and @randyspiess for your contribution.

However, our CI / CD tests (running in Travis) are failing. Would you be able to correct this too? Looks like openjdk7 and 8 are no longer available in Travis.

Once Travis is passing, I'm happy to merge these changes as I can see in the Pluggable SCM provider code base here where the gerrit provider trigger methods lives it also appends "plain:" which would result in plain:plain:master.