Fix deprecation warnings being emitted

~arash/luigi/parameter.py:168: DeprecationWarning: Use has_value rather than has_default. The meaning of "default" has changed
  warnings.warn('Use has_value rather than has_default. The meaning of "default" has changed', DeprecationWarning)

I verified that it works for me by running a normal Spotify luigi job.
Merge pull request #431 from Tarrasch/fix_has_default_deprecation

Fix deprecation warnings being emitted