Fix deprecation warning

     INFO:subp.spluigi:/home/arash/spotify/repos/luigis/github_luigi/luigi/parameter.py:190: DeprecationWarning: Use value rather than default. The meaning of "default" has changed
     INFO:subp.spluigi:  warnings.warn('Use value rather than default. The meaning of "default" has changed', DeprecationWarning)

I verified this by running a normal Spotify luigi job.
Merge pull request #441 from Tarrasch/default_deprecation_warning

Fix deprecation warning