Freshen fixtures only when necessary

Now the rake-task `test:freshen_ontology_fixtures` actually only *freshens* - with *freshens* being used in the sense of updating the hets-output of an ontology only if the hets-version is newer than the one which generated the old version.

Additionally it'll return travis (and us) to the default-rake task. So we can just run `rake`. If necessary it will
update or create the fixtures files and then run the tests.