Revert AbstractDiff compat to 0.5 (#126)

Author: gdalle

Project.toml

@@ -26,7 +26,7 @@ ImplicitDifferentiationStaticArraysExt = "StaticArrays"
 ImplicitDifferentiationZygoteExt = "Zygote"
 
 [compat]
-AbstractDifferentiation = "0.5, 0.6"
+AbstractDifferentiation = "0.5"
 ChainRulesCore = "1.14"
 ForwardDiff = "0.10"
 Krylov = "0.8, 0.9"