Update to work with HARK 0.13.0

[sbenthall]

Tests are failing currently because of changes to HARK master -- which will soon be HARK 0.13.0

The most obvious issue has to do with the change of Distribution.approx to Distribution.discretize

[alanlujan91]

This gets addressed in #101 although if it takes longer to merge that PR we can make a specific change for this issue