(#17713) make upstart_version an instance variable

This fixes the bug in some later versions of ruby which causes warnings
to be thrown when this class variable gets set from an instance of the
class.
Merge branch 'adregner-fix/master/service_upstart_class_variable_access'

* adregner-fix/master/service_upstart_class_variable_access:
  (#17713) make upstart_version an instance variable