Unity Engine has been upgraded to 5.4.0p4 - This is a patch version. Go to the Unity site and "get Unity" then go to the patch releases page.
KSPUtil.dll has been merged into Assembly-CSharp.dll - change references in your Project.
A LOT of redudant classes that were not being used any more have been removed.
KSP now uses 9.80665 and 6.67408e-11 instead of 9.81 and 6.674e-11 for gravity equations.
KSPedia and asset