Added compatibleSinceVersion

To display warnings like: https://wiki.jenkins-ci.org/display/JENKINS/Marking+a+new+plugin+version+as+incompatible+with+older+versions#Markinganewpluginversionasincompatiblewitholderversions-ModificationtoDisplayofUpdateablePluginList

Though the Wiki page says that I need use "compatibleSinceVersion" when automatic data format upgrade can't be applied, the warning text looks proper for this case.