A couple years ago, Unity said they would be upgrading to .NET 4.0 Framework soon. That day has yet to come.
Until that day, you can use this .NET 2.0 version of the Leap SDK to integrate with Unity. Just drop the .dll file into the assets/plugins and you're good to go!
Please report any bugs, Leap hasn't released the source code of the newer versions of the SDK, so I had to reverse engineer some of it.