refactor freshen-fixtures task for readability.
extract hets-cmdline-options into config.
freshen fixtures only when necessary

'Necessary' is being determined by the modified timestamp of the
hets-binary (or wrapper-script) in relation to the xml-file which is to
be produced.
add freshen-task to default rake-task

And rely on this task when running travis, again.
fix default rake task

The default rake-task comes with spec and cucumber as dependencies. If
we provide more we just append to this list. So we'll clean the
prerequisites of those tasks beforehand.
Merge pull request #1096 from ontohub/freshen_fixtures_only_when_necessary

Freshen fixtures only when necessary