The latest version of Orion's Core Assets is 4.1.6, found here: https://developer.leapmotion.com/unity#100
Try that version for working with Unity 5.6. Let me know if you're still getting the same errors; We are currently running on 5.6.1f1 successfully as far as we're aware.
Be sure to delete any old LeapMotion or UnityTestTools folders before you import a new version of the Core Assets.