robotstxt: addon-repository JAR manifest does not have Implementation-Build number

This defect causes WARN log messages during CMS repository initialization:

WARN [org.hippoecm.repository.LoadInitializationModule.locateExtensionResources():672] Build number not valid, not storing it on initialize item 'robotstxt'. Is Implementation-Build manifest entry missing?

the repository-addon should be using the buildnumber maven plugin to put the Implementation-Build line into the manifest.